The Church has been called to form consciences not to replace them' (Pope Francis, 2016 Amoris Laetitia 37). The Catholic tradition teaches that a person is obliged to follow his or her conscience. This teaching takes into account the way in which the individual's desire for what is good, and his or her capacity to reason about what is good and right, and choose freely to do what is morally right, are related to the idea of objective moral truth. Though we frequently talk about conscience as if it were a thing, it is really an abstract construct that is used to talk about how we engage morally with the world. Conscience, literally translated from its Latin root, means with knowledge. At its simplest level then, to act in good conscience, or to follow one's conscience, is to act in way that is in accordance with the knowledge that one has about a particular situation.
From a Catholic perspective, only actions that are willed (i.e., consciously and freely chosen), the so-called actus humanus, are able to be evaluated in moral terms. That one freely chooses to act in a certain way is what makes an action a moral action. However, the quality and degree of knowledge on which the choice to act was based is important in the evaluation of the action as morally right or morally wrong and in evaluating the moral culpability (i.e., responsibility which may entail guilt or blame) of the person who chooses to act in a particular way. It is possible, therefore, to do the right thing for the wrong reasons, or to do the wrong thing for the right reasons. To examine one's conscience is to examine one's actions, together with the reasoning that prompted those actions and the personal judgments that one made.
In any given moral situation, there is always a morally right answer; there is always a choice that would be the morally right one. There is, in other words, an objective moral truth, the moral truth. But we have to understand all this very carefully, since it is quite possible that there may be two or more objectively good things that we can choose to do, and since we are free beings, making a choice for either would still be morally right. Given the complexity of most moral situations, the challenge is in knowing what that moral truth is. A large number of variables come into play. Among such variables are the kind of person you want to be, the various competing good things that need to be taken into account, the possible harms, the outcomes desired and the outcomes foreseen though not desired, and of course, the circumstances and the consequences that one cannot foresee. Human beings rely on their individual and collective experience (of which law, tradition and divine revelation forms a part) together with their feelings, and their reasoning ability, to work out what that moral truth is in any given situation. Conscience is the means through which we relate to objective moral truth. The exercise of conscience includes the capacity to perceive which values (goods) and harms (evils) are at stake, the process of reasoning about those values and the ways to achieve them together with judgment about which values and which way to achieve these values is is the morally right one. Conscience understood as the desire for good, the aggregated knowledge of experience and reason about what is good, and the judgments made about what is the good and right thing to do) is the individual's relationship to the objective moral truth. Assuming it was possible for you to have perfect knowledge of any given moral situation, then, provided you always chose to act in accordance with that knowledge, your conscience would be aligned with the truth and would always be perfectly good and perfectly right. To have perfect knowledge would be to know everything about all of the factors involved. Such perfect knowledge includes all the facts (scientific and otherwise) of the matter and all of the circumstances and foreseen and unforeseen consequences. Perfect knowledge also assume that one knew perfectly what all the values (goods) were at stake in a given situation and which were the most important values in that situation (e.g. freedom vs security). Having perfect knowledge would also include the most morally relevant parts of the action (who you want to be by doing this, what you do, and what it achieves), which behaviours are always morally bad because they attack the fundamental good of the human person, and the perfectly correct approach to moral reasoning about all of this. Clearly, such perfection can be attributed only to God. We know, from our own experience, that humans can never have perfect knowledge and hence cannot achieve perfect goodness. But, they can come close to it. That is because they are made in the image of God, and thus have been gifted with the capacities of reason and freedom, and are social beings who can learn from each other. When these capacities and our social awareness are correctly combined, we can acquire proper, if limited, knowledge. We can make moral decisions that we believe to be morally right based on that knowledge. In other words, in moral matters, we can only have 'moral' certainty in our judgments. This means we make the judgment about what we ought to do, or avoid doing, without any well-grounded fear that we are wrong or mistaken in that judgment. In this process, the virtue of prudence, namely, the habit of wise judgment is central. We need also to keep in mind that, through our sharing in the divine life, we are helped by the Holy Spirit in our decisions. The gift of wisdom assists us to see, judge and respond as God would. Thus we are called to collaborate with God in our moral life.
Since conscience is your own relationship to the objective moral truth, you are obliged to follow your conscience. In other words, what one determines to be good and right based on the use of one's reason is like a law that must be obeyed (Gaudium et spes, 16). It is the closest approximation one has to the truly good. It is the way that one participates in God's goodness. It is here that one is 'alone with God, whose voice echoes' in one's depths (GS 16). So, to not obey your conscience, to choose not to do what you know to be the morally right thing to do or to blindly obey others is to abdicate personal responsibility for moral decisions. to act in this way is a sin because it is tantamount to idolatry. If conscience is where we relate most closely at a personal level with the objective truth of God and perfect goodness, then to do other than what we believe to be right and true in our conscience is to do other that what we believe to be God's will, to do other than what we believe to be perfect goodness. That is the essence of what we mean when we talk about sin.
Since conscience is our relationship with the truly good, and so with God who is true goodness, each person should want to form a correct conscience. Of course, it remains possible for a person to act in good conscience, sincerely believing that what he or she is doing is morally right, when in fact the action is morally wrong. The Catholic perspective maintains that sincerity is really important since a truly sincere pursuit of the good and the right can always grow to fuller knowledge of the truth with consequent changes to moral decisions and behaviour. This is especially the case when one makes the wrong decision in good conscience believing it to be the right decision because one did not have the necessary knowledge to make the right decision and could not, given the circumstances, be expected to have the necessary knowledge. The term to describe this situation is 'invincible ignorance'. When full knowledge could not be expected, or was not possible, there is a mitigating effect on the moral culpability of the person who made the decision, because the action cannot be described as being truly voluntary. So, whilst what might have been done may still have been objectively morally bad, we would not say that the person intended it to be so, since he or she could not have been expected to have chosen otherwise based on the knowledge at his or her disposal. 'Vincible ignorance' on the other hand, is when someone who could legitimately be expected to know better nonetheless chooses to remain ignorant, or does not make a reasonable effort to determine what the morally right thing to do would be. For example, a seventeen-year-old is obliged to know, and indeed should want to know, about what is morally right when it comes to the practice of sex and the reasons why the Catholic perspective maintains that the only morally good place for sex is in marriage. Moreover a seventeen year old should also know the personal risks and the risk to others of having unprotected sex with multiple partners. Everyone should want to know how our values and seemingly innocent choices about matters such as what we purchase, what we eat, who we vote for contribute to suffering and injustice in the world. To choose not to know, or to ignore what one does know, are both morally irresponsible actions. That said, the complexity of many of these situations and of simply getting on with living life in today's world, mean that there are always limitations to what we can really know.