The reason that we should not pray for one in a state of mortal sin (the ONLY time we are told NOT to pray for someone) is because it will do no good.
Really? I have never heard of such a thing – nobody has ever “told” me not to pray for someone depending on the state of their soul. Didn’t St. Monica pray for her son, St. Augustine, for all those years while he was committing every grave sin under the sun before he became Catholic? Isn’t that the primary reason why she herself is a saint?
Also, how can you possibly “know” if someone else is in a state of mortal sin? Only God and a sinner’s confessor would have access to such knowledge. It seems to me to be very dangerous to start judging the state of other people’s souls.
And since I just referenced St. Augustine, here is one of my favorite passages from his writings regarding mortal vs venial sins:
“Let no one say: ‘I did that; perhaps I will not be forgiven.’ Because you did it? How great is the sin you committed? Tell me what you have done, something serious, something horrible, something terrifying even to think about. Whatever you might have done, did you kill Christ? There is nothing worse than having done that, because there is nothing better than Christ. How great a wrong is it to kill Christ? But the Jews killed Him; and afterwards many of them believed in Him and drank His Blood: and the sin which they had committed was forgiven them. When you shall have been baptized, keep to a good life in the commandments of God, so that you may preserve your Baptism to the very end. I do not tell you that you will live here without sin, but they are venial sins which this life is never without. Baptism was instituted for all sins; for light sins, without which we cannot live, prayer was instituted. What does the prayer say? ‘Forgive us our debts as we too forgive our debtors’ [Matt. 6:12]. We are cleansed only once by Baptism; by prayer we are cleansed daily. But do not commit those sins on account of which you would have to be separated from the Body of Christ ; perish the thought! For those whom you see doing penance have committed crimes, either adultery or some other enormities: that is why they are doing penance. If their sin were light, daily prayer would suffice to blot them out. In the Church, therefore, there are three ways in which sins are forgiven: in Baptism, in prayer, and in the greater humility of penance; yet, God does not forgive sins except to the baptized” (Sermon to Catechumens, On the Creed [ca. 395 AD]).
