Considering the manner in which conscience obliges us to follow it, we should consider moreover the question, does a doubtful conscience oblige us?
It seems that it does:
1. For conscience is a matter of good or evil in reference to the ultimate end, pertaining to salvation or damnation. But, to use a classic example, if a hunter, in the forest hunting deer, sees a large moving object, but is in doubt about whether it is a deer or a human, he must not shoot, until he is sure that it is not a human. Even more so, must one in doubt regarding a potentially sinful matter have certainty before acting.
2. Again, if someone has a doubtful conscience about doing something, he believes that doing it might be wicked. Therefore, if he willingly does it anyway, he seems to show that he is willing to do a wicked action, as through thinking “this might be wicked, but I am going to do it anyway.”
3. Again, St. Paul says, “Anything that is not according to faith” which here seems to mean conscience, “is sin.” (Rom 14:23) Therefore what is done aside from or against a doubtful conscience is sinful.
4. Furthermore, if reason is still uncertain, yet someone proceeds to act, it must be emotions or brute will that determines what it he does, but this pertains to precipitation and is a form of imprudence. Therefore it is sinful to act while reason is still uncertain about what is right and wrong, or good and bad.
5. Further, St. Thomas Aquinas says, “it is dangerous to resolve (determinare) a question about mortal sin, if the truth is not evident (nisi expresse veritas habeatur); for error by which that which is a mortal sin is believed not to be a mortal sin, does not excuse conscience altogether, though it may partially excuse it. (non excusat a toto, licet forte a tanto)” (Quodlibetale 9, q.7, a. 2) But a doubtful conscience presupposes that the truth is not evident; if it were evident, one would not be in doubt about the matter. Therefore a doubtful conscience obliges, at least when the doubt concerns something that is potentially a mortal sin.
6. Finally, St. Alphonsus Ligouri teaches outright: “It is never licit to act with a practically doubtful conscience; and in a case where someone acts he sins, with a sin of the same sort and gravity as his doubt concerns, because someone who exposes himself to the danger of sinning, already sins according to what is said in the Scripture, “He who loves danger will perish in it.” (Sirach 3:26.) (Theologie moralis I.1, 22), The same thing is taught by many thomists and moral theologians (e.g., Edward Feser, Charles Coppens in Moral Theology, Fr. John Hardon, Catholic Dictionary, “Rules of Conscience”)
Sed contra 1. Against this, St. Francis de Sales recommends as the chief rule of obedience “Let everything be done for love, nothing for fear. Love obedience more than you fear disobedience.” (Emphasis by capitalization in original). But if one foregoes doing what seems good because one is uncertain if it is wrong by reason of violating some legitimate rule, one seems to be acting more out of fear than love. Therefore a doubt of conscience does not prohibit one from pursuing what seems to be good, even when one has some fear of its being wrong, which is to have a doubting conscience.
Sed contra 2. Again, it is sometimes impossible for one to dispel all doubt about whether one should do some action or refrain from it. Therefore, if acting with a doubtful conscience were sinful, one would be unable to avoid sin, which is an unreasonable position. Therefore acting with or on a doubtful conscience is not sinful.
Sed contra 3. Again, certainty is not to be sought equally in all things, but inasmuch as the matter permits. But in some matters it is not practicable, and therefore not prudent, to seek certainty. This may apply either because the matter is so obscure or complicated that even long deliberation will not attain to certainty, or because one must act quickly, and in a short space of time it is not possible to dispel doubt about whether acting or not acting is the right thing. To take another classic example, if you are asked by the Nazis if you are hiding Jews, and are unsure whether you should tell the truth, lie, equivocate (if a suitable equivocation occurs to you), or say nothing, taking more than a brief moment to think about it may be equivalent to saying nothing, resulting in a search. But what cannot be avoided (acting or refraining from action with a doubting conscience) cannot be blameworthy. Therefore, a doubtful conscience at least does not always oblige.
Sed contra 4. Again, doubt of conscience may pertain not only to doing A or B, but to doing A or not doing A. And sometimes the doubt may be greater on the side of not acting, i.e., it is more probable that not doing A is sinful than that doing A is. But if it is wrong to act on a doubtful conscience, it seems that one would rather be obliged to not act, and therefore to what is more probably sinful.But this is an unreasonable position.Therefore, again, one is not obliged by a doubtful conscience.
I respond that the first and fundamental precept of natural law is “do good and avoid evil.” This obligation is manifested and realized in the obligation to particular goods. As rational beings, we do the good and avoid evil not by mere natural necessity, but by choice and judgment. Since we choose good (or evil) in accordance with our apprehension and judgment of the good, we are obliged to pursue and hold fast to the truth about what is good, so far as this is reasonably possible and necessary in order to choose the good.
I say “reasonably possible and necessary” because it may be more or less difficult for us to determine what is good or evil, right or wrong. Again, in some matters it is of greater importance what we do, in other matters of lesser importance. To what extent deliberation is reasonable will depend on the degree to which one might, through further deliberation or consultation, arrive more surely at the truth of the matter, as well as on the importance of the matter.
Sometimes it is difficult to discern what is good because we do not know some relevant facts. For example, one considering whether he should attend Mass while ill may not really know what the risk of him infecting someone else is, or what the chance is that a immunocompromised or otherwise vulnerable person could be present and possibly end up being infected by oneself.
In other cases, the facts are more or less clear, but how moral principles should be applied is not so evident. For example, returning home after shopping at the supermarket, one might discover that some item was by an oversight not rung up by the cashier. Is one obliged to return to the supermarket to pay for the item? Or is the burden of doing so out of proportion to the value of the item and therefore one has no obligation in justice to do so? Or because one might equally another time be doubled charged yet unable to be refunded for the double charge (on account, say, of a store policy), is it fair to keep the item this time without having paid for it?
Similarly, some matters are of great importance, and justify a great degree of deliberation, others are of less importance, and do not justify much deliberation; protracted deliberation might, indeed, detract from other good things one could be doing, and such deliberation might therefore itself be wrong. A potential move from California to Texas might be worth much deliberation. When making a cake, whether to put half a cup (to reduce calories) or three quarters of a cup of sugar (to make it sweeter) into the cake, is not worth deliberating a long time. Similarly, whether abortion is the killing of an innocent human being is worthy of considerable deliberation (so far as one has not previously attained clarity on the issue, so that no new deliberation is necessary), but whether in a specific instance, here and now giving a beggar on the street a dollar is reasonable and just (because it helps him in his need), or bad (because such economical support without other means of support only furthers the systemic problems in his life and society, or because it may be supporting organized crime), is not worthy of long deliberation (though the question of one’s attitude to begging and beggars in general may be worthy of considerable thought).
Consequently, in matters of lesser importance and/or when any additional confidence acquired through further deliberation does not justify the added effort and time, one is not obliged to settle doubts of conscience.
We should, therefore, say that a doubting conscience does not in principle oblige a man to avoid doing what he is afraid might be wrong, or to do what he is afraid might be obligatory. Nor does it always oblige him to resolve the doubt. And so we conclude that a doubting conscience does not, as such, oblige.
But as the arguments on the other side, that a man must not act on a doubting conscience, are not without weight and this opposite position is commonly held by moral theologians, we should take note of a certain truth in this position.
When a man reflects on his obligation or lack therefore to further seek the truth, he may often thereby resolve the doubting conscience, if he comes to surety about what he should do in this situation. There are, indeed, three outcomes of this self-reflective deliberation about whether he need deliberate further.
1. He may see that he is obliged to do what he can to resolve his doubt;
2. He may see that, because of the difficulty in resolving the doubt, or because of the minor importance of the matter, he is under no obligation to resolve the doubt, and indeed might be guilty of wasting time or of scrupulosity in pursuing the matter.
3. He may be unable to decide whether he ought to try to resolve the doubt, or just act according to his best lights. This happens especially, though not exclusively, with persons inclined to scrupulosity.
In the first two cases, his conscience is no longer doubting, but sure: in the first instance, that he ought to resolve the original doubt; in the second instance, that he should act according to his currently best judgment; in the third case, however, the doubting conscience remains, and one has no choice but to act with a doubting conscience (refraining from action to which one might be morally obliged is considered as an act, morally speaking).
Thus, a doubting conscience may in some cases be resolved to a sure conscience through this self-reflective consideration. It is, however, not always possible to resolve the doubt in this manner. And when it is possible, it is not always reasonable or obligatory to do so, when e.g., the issue in question is so minor that even the additional self-reflective act by which one might explicitly decide that one should act despite one’s doubt is superfluous or excessive.
Sometimes, indeed, a man may be obliged to such reflection, when the matter is important enough to require at least the further (self-reflective) deliberation of explicitly considering whether one should devote more resources and time to (objective) further consideration. And in this regard the arguments that a man must not act with a doubting conscience, but must resolve it, have some truth behind them. However, the obligation to further pursue the truth of the matter does not derive from conscience itself, but from the importance of the good in question. Again, this resolution of doubt to certainty is not always possible, and, even when it is possible, a person is not in every case obliged to attain this subjective and self-reflective certainty. And so it is not true unconditionally.
Ad 1. The first argument to the contrary is valid for those situations where the matter is important and further consideration can resolve the issue, but fails to conclude universally. For example, in a country wrought by civil strife over the government, a warden or executioner might be unsure whether a convicted traitor is actually a traitor, or whether the conviction is politically motivated and unjust. He may rightly act according to his best judgment, even while fearing that he is either cooperating with injustice or disobeying legitimate authority.
Ad 2. To the second argument it should be said that this argument assumes something false, namely that the person who does something that might be bad does not care if it is, whereas it may be merely that he is not reasonably able to clarify the matter.
Ad 3. To the third objection it should be said that St. Paul is here discussing the situation of those Christians who believed that they were obliged to avoid eating meat. Thus, we should understand “not according to faith” or “conscience” in the sense of “contrary to conscience”, and so St. Thomas Aquinas also interprets this passage in his Commentary on the Letter to the Romans: “He who “distinguishes”, i.e., he who has a false opinion that he must distinguish foods, namely the foods he considers to be illicit, is condemned, if he eats…. Because it is not from faith, but is rather against the faith, i.e., against the truth of faith and the conscience of the one who does it.
Ad 4. To answer the fourth objection we should note that we naturally and normally do many things without an explicit and complete argument for why it is good to do them or better than their alternative, acting out of habit and various spontaneous inclinations. For example, when shopping, I might choose cherries rather than grapes, because cherries taste better, or grapes, because they are cheaper, without actually considering the potential argument in favor of the other one. It would be unnatural and indeed impossible to reason everything through step by step at every moment.
Thus, when reason perceives something good yet retains some doubt about it, if the situation does not oblige a man to resolve the doubt, as we explained above, it is not contrary to reason, and in some cases might even be the most reasonable thing to do, to do that to which he is spontaneously inclined, or to follow his intuition, to use the common language.
Ad 5. To the fifth objection we should first note that Aquinas does not give an argument for why he on this issue states that error about a mortal sin does not excuse conscience altogether. It is difficult to reconcile this claim with what he says about ignorance (that an act that follows from involuntary ignorance is itself to be considered involuntary), except on the supposition that any error by which that which is a mortal sin is believed not to be a mortal sin, is at least partially voluntary. As this position stands in opposition to experience, particularly when considering the influence of parents and the surrounding society on the moral judgments of children, and as it inclines to a presumptuous judgment of the conscience of others (since when we judge their judgment about a grave matter to be wrong, we would presume at least venial sin on their part), we should not assent to it. We further note that this argument would apply not merely to a conscience holding something to be probably legitimate, but to a conscience erroneously holding something to be certainly legitimate.
We may, nonetheless, grant the argument to this degree, that in grave matters, when it is not of serious importance whether or not we act (which implies that we have no doubt that it is morally legitimate not to act), we should avoid acting when we have serious doubt that it would be gravely wrong to act.
Ad 6. To the sixth we reply that St. Alphonsus Liguori and many moral theologians have desired to create a system capable of resolving all moral cases, and consequently tend to reduce “gray”, or areas of indistinct truth, to black and white clarity. Indeed, some cases of doubt can be resolved by application of the principle that “a doubtful law does not bind”, as explained above, and as St. Alphonsus goes on to explain. But the implicit assumption that it is always possible, reasonable and obligatory for someone to resolve his doubt, at least by application of this principle, seems neither to be based on empirical experience, nor a logical argument, but serves to help establish such a system by which every moral case can be clearly and distinctly resolved. This assumption, as noted above, is untrue. More fundamentally, the desire lying behind such theories, the desire to establish a system capable of logically resolving all moral cases does not correspond to moral reality, which is measurable not so much by a logical system as by the virtue man.
The arguments on the other side may be granted, the distinctions or qualifications one might make in regard to these arguments being left as an exercise to the reader.
I have a question about this in regards to my own experience: At Mass, I was concerned that I had swallowed a piece of food that was in between my teeth because I didn't know if that broke the Eucharistic fast. I didn't think that it broke the fast, but I had doubts and didn't know for sure. I still received the Eucharist, but after Mass I was worried about it. I looked it up and I found out that it didn't count as breaking the fast since it wasn't purposely to eat food, but I didn't know that at the time. Would this be gravely sinful, since even though the action itself wasn't grave I did it anyway with doubts about whether it would break the fast or not, since I knew that breaking the fast was grave matter? Thank you!
Since what you thought at the time might have broken the fast was both minor, i.e., concerned a tiny piece of food, and not deliberate, your decision to receive Communion despite this doubt seems to have been quite reasonable, and not a sin at all, mortal or venial.