"If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this. 17 All unrighteousness is sin, and there is a sin not leading to death," (1 John 5:16-17).
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What this verse is referring to is the distinction between venial and mortal sin and the regards to the efficacy of prayers. Venial sin is not mortal in that it does not kill the soul, mortal sin is called that because it is the sundering of the soul from God out if its own will and thus it is dead.
Venial sin can be remitted by God out of mercy, and thus we pray for sinners to God that he may show mercy on them, so that the purifications neccessary for one possessed of venial sin to enter heaven are lessened by acts of grace. This is possible because even one who has commited venial sin, if he has not commited mortal sin on his death is still saved, he has already made the choice to choose God and thus God will intervene to lessen the purifications as he is not biolating free will.
However in regards to mortal sin, only the individual will of the soul moved through repentance and God's grace can make the free choice to return to the Lord and giver and life, and be raised through repentance from the death of sin, into the life of God. Thus in this case prayers for divine mercy on one who has commited mortal sin are useless because only the sinner in such a state can make the act to return to God to be remitted his sins.
We can still pray for all sinners as we do not know (as no man can truly judge) whether a man has indeed sinned mortally. Likewise we can also pray for one likely to have commited mortal sin while he is alive in the hope that they may listen to God's grace and repent. However prayers to God for leniency and to forgive his mortal sin are futile because only the sinner who has commited mortal sin can make the choice to repent as he has broken from God in a free act of will, and since God respects free will to ensure a true relationship with man he will not force someone to choose Him over sin.