Conscience is, therefore, a voice springing from the whole being. It is partly a judgement on principles, as when my conscience tells me that such and such a principle is wrong, and partly an application of principles, as when in the ordinary round of events I reject a temptation to do something because my conscience informs me that this would not be right, would not accord with certain principles that my faith has taught me to accept. On the whole, and chiefly, it concerns the application of principles rather than the mere selection of principles, for these are selected by the reason or the instinct, or the light of revealed truth. In the voice of conscience, then, we notice the idea of moral obligation of moral insistence: the root idea of it is, "I must." This voice of conscience, then, we certainly have to obey, for it is the sole personal command that reaches us. Even authority could not be accepted nor its ordinances respected unless it had been backed by the full majesty of conscience. Conscience must sit in judgement on the claims of authority before investing it with the sanction of the moral law. A Catholic has first to convince him self of the divine mission of the Church and be sure that she represents the teaching body that our Lord came on earth to found, before he can allow her to make with effect any demands upon his allegiance. Either deliberately or by implication he has to be made sure of his ground by conscience. So, again, in every action in which the moral obligation which we summarize under the name of duty is felt and attended to, I have to convince myself of the authority of conscience, and have to put conscience in judgement over the claims that are made upon me.
But while in this way I am completely under the dominion of my conscience, I have to remember that, in consequence, I cannot move until my conscience is sure. I may not act until my conscience is really determined: I cannot act, that is, when my conscience is in doubt. The reason of this principle is that, were I to do so, I should in effect be saying to myself, I don't know whether this is right or wrong, but I am going to do it anyway. Obviously this would be altogether a disrespectful attitude to God, a complete disregard for the law of God. Yet on the other hand, it is surely very difficult to make up one's mind determinedly on all the points that have to be settled by conscience? Surely, at least on the spur of the moment, it is almost impossible to be certain? Often I have to admit that I am not quite sure, but that I think a certain thing is allowed. And here am I doing wrong, for I am acting on a doubtful conscience? No. Why? Because really and practically my conscience has been made certain. What has happened is that I have put myself into some such position as this: I have said I must act from a sure conscience, but in this particular matter I am not quite certain what is right. However, it seems to me that under the circumstances I have enough to justify my doing it, for I do really think it to be allowable; hence I have done the best I could under the circumstances, for if I were certain that the thing I was going to do were wrong, I should, of course, not have done it. But as I must act somehow, and as this does not appear to me to be actually wrong, I am justified in going through with it. In this way by a reflex act, by getting as it were behind my conscience, I have in reality made my conscience sure, and can proceed to act on it.
I have, then, just to do my best, for my conscience is infallible: that is to say, if I make up my mind seriously that a certain thing is right, it becomes right for me. My conscience is not infallible, of course, in the sense that whatever I think right is right in itself, but only that it is right for me. Supposing, for example, that when I am quite a boy I think that I have a vocation to the priesthood, or at least I think it possible that God may have destined me to be a priest, and supposing, further, that after considering it carefully praying, asking advice, looking at my capabilities, my gifts, my circumstances I come to the conclusion that I have no such vocation, what is to happen if I find out afterwards that God did intend me really to have been a priest? Nothing will happen at all, and God Himself will applaud my action in giving up the idea of the priesthood, for I am conscious that God can only ask me to do my best. He cannot expect from me absolute perfection, for He knows (since He made man) that all that I can do is unprofitable. All that He can with any right expect is that I should try to do the right thing. But I have tried; I have prayed, sought counsel, considered the matter: then I have acted as I honestly judged best, and I must trouble myself no more about it. Even when other people tell me what they think I ought to do, even when the priest gives me advice in the confessional (unless I have been rendered abnormal and incapable by scruples), I have finally to remember that with me and my conscience lies the ultimate responsibility of it. I may not plead their words in my excuse, for my soul is my own. Guided by conscience, which itself has been trained by faith and the moral law and by the example of Christ's life as I find it in the Gospels, I have to steer my own way.
- text taken from by Father Bede Jarrett, O.P.