Today we read the last chapter of the first letter of John. In it John tells us to pray for one another if we observe another sinning.
If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. 17 All wrongdoing is sin, but there is sin that does not lead to death.
1 Jo 5:16-17
Here we see John articulating two types or degrees of sin. There is lesser type or sin and there is sin “that leads unto death”. This second type or sin became known as “mortal sin” and the lesser sin is known as “venial sin”. Notice, John says that venial sin can be rectified by prayer alone. However, this implies that mortal sin cannot be rectified solely by prayer, something more is required. This is where Jesus commission to the Apostles, “whose sins you forgive are forgiven, whose sins you retain are retained.” It is the Apostles who have authority to forgive these mortal sins. Now obviously, the nature of sin does not change after the death of the last Apostle. Thus, there must be some way for this commission to continue after the death of the last Apostle. That method is the ordination of bishops by the laying on of hands in a line of Apostolic Succession.
Over the years the Church has defined a mortal sin as:
Mortal sin is a sin of grave matter – (it must be of a serious nature).
Mortal sin is committed with full knowledge of the sinner – (you must know that it is a sin).
Mortal sin is committed with deliberate consent of the sinner – (it must be your intent to do it).
Thus, contrary to popular belief, one cannot “accidentally” commit a mortal sin. To commit a mortal sin one must intentionally do something that they know is seriously wrong. We can see immediately why we are forever incapable of judging the state of someone else’s soul. Has the person who steals bread to feed his family done something that is seriously wrong? Contrast that with someone who scams elderly folks out of their life savings. Ultimately, you conscience is your judge as to whether you have committed a serious enough sin with the proper knowledge and intent.