Many people believe that this sin leading to death in 1 John 5:16 is suicide. I don't personally believe that. I think this is a reference to spiritual death and blasphemy of the Holy Spirit
I believe that 1 John 5:16 is speaking in far more practical and literal terms. I believe an example of "a sin leading to death" that we should not pray for is presented in the story of Ananias:
Acts 5:1-6 NASB
1 But a man named Ananias, with his wife Sapphira, sold a piece of property, 2 and kept back some of the price for himself, with his wife’s full knowledge, and bringing a portion of it, he laid it at the apostles’ feet. 3 But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back some of the price of the land? 4 While it remained unsold, did it not remain your own? And after it was sold, was it not under your control? Why is it that you have conceived this deed in your heart? You have not lied to men but to God.” 5 And as he heard these words, Ananias fell down and breathed his last; and great fear came over all who heard of it. 6 The young men got up and covered him up, and after carrying him out, they buried him.
Clearly there can be no doubt that Ananias' sin led to his death.
Furthermore, I believe that John is simply being practical in pointing out that there is no point in praying for someone after God has already passed the ultimate judgement on the case. To whom would one appeal God's perfect decision?
Thus while a sinner still has life, there is still hope that God will spare them, but once God passes his judgement and calls them to face Him, prayer is pointless (with all due respect to my Catholic brothers and their beliefs on purgatory).
I see another example of this attitude in the death of the son of David and Bathsheba:
2 Samuel 12:21-23 NASB
21 Then his servants said to him, “What is this thing that you have done? While the child was alive, you fasted and wept; but when the child died, you arose and ate food.” 22 He said, “While the child was still alive, I fasted and wept; for I said, ‘Who knows, the LORD may be gracious to me, that the child may live.’ 23 But now he has died; why should I fast? Can I bring him back again? I will go to him, but he will not return to me.”
