What Is the Meaning of ‘Sin Unto Death’?

If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. (1 Jn, 5:16)

This passage from Scripture is the basis for the Church’s distinction between mortal and venial sin (Catechism, 1854). The Church teaches that venial sins hurt but do not sever our relationship to God whereas mortal sins do sever our relationship to God. A sin that severs our relationship to God who is life itself is rightly called “deadly.”