Common sense and divine revelation agree that not all sins are equally serious. Differences in their seriousness arise from differences in what is done, in the awareness of wrongdoing, and in the appeal of a morally acceptable alternative. The more serious the matter, the clearer the judgment of conscience, and the easier the choice of a morally acceptable alternative, the worse the sin.
The significance of the worst venial sin is altogether different from that of the least serious mortal sin. Faith requires this sharp distinction between venial and mortal sin—a distinction which is clear in Scripture and the teaching of the Church.
A mortal sin deprives one of divine life; mortal sinners exclude themselves from God’s kingdom, separate themselves from Christ, and evict the Holy Spirit from their hearts. The conditions required for mortal sin are grave matter, sufficient reflection, and full consent. Certain kinds of acts are of themselves light matter, while other kinds are of themselves grave matter. Even among the latter, gravity sometimes can be lacking due to the smallness of the harm done. Sufficient reflection requires awareness at the time of decision that the act is or may well be gravely wrong. Full consent is a definite choice.
Individuals can become guilty of mortal sin by involvement in a group which acts in a seriously wrong way. But the responsibility of individual members cannot extend beyond their power to affect the group’s actions. Similarly, although individuals have a responsibility to rectify the group’s wrongdoing, this responsibility is not absolute: The norms for resolving conflicts of duty apply. Likewise, even when it would take only inaction to avoid contributing to the group’s immoral action, the individual’s responsibility is not absolute, provided the socially required act can be done for some good. For example, taxes are used for evil purposes and also for good purposes; citizens may be justified in paying their taxes and even obligated to do so, in order to support the good purposes and avoid imposing heavier burdens on others by the nonpayment of taxes by which they might otherwise seek to avoid involvement in the society’s wrongdoing.
Sins of thought are very important to Christian morality. Scripture and the teaching of the Church testify to this. Evil thoughts are important not only because they are the starting point for evil deeds but, more profoundly, because morality primarily pertains to the mind and the will. Evil is much more in the heart than in outward deeds; its greatest moral significance does not lie in the harm done by outward behavior but in the privation in the existential domain. Legalism and pharisaism are natural consequences of attempting to avoid sinful behavior while freely committing grave sins of thought.
Sins of thought must be distinguished from what precedes them. Spontaneous emotional reactions are not themselves sinful (although, in the fallen human condition, even normal emotion has the character of concupiscence, and each individual’s emotional reactions often reflect a further lack of integration arising from upbringing and personal sins). Nor is spontaneous deliberation about an immoral course of action sinful, since proposals for choice naturally come to mind when one confronts a situation, and one naturally begins to consider their good and bad aspects.
However, in beginning to deliberate about an immoral proposal, one is in a situation of temptation. Also, thoughts, desires, and experiences of satisfaction arising from some prior sin are themselves sinful—though only venially so, unless they arise in implementing a prior, mortally sinful choice. Where prior, unrepented mortal sin does not play a role, however, mortal sins of thought begin only when the usual conditions are fulfilled: grave matter, sufficient reflection, and full consent. Choice is a conscious act, and a person uncertain of having made a choice a moment before can be sure he or she did not make it. Still, unreflective people can and do make choices without being able to point to anything in their experience which they would call a “choice.”
The primary case of a sin of thought is a sinful choice. There is sin here whether or not the choice is carried out, though carrying it out makes the sin worse. Evil wishes are also matter of sin, as is considering with satisfaction and approval the doing of evil, whether by oneself or another. However, it is possible to enjoy knowing about evil and to approve good consequences of evil without taking satisfaction precisely in the evil itself.