The Hopelessness of Apostasy
Given what John has already said about the antiChrists and those who follow them into the world (outside of the fellowship with the Son and Father), the breakdown of verse 16 goes something like this:
16a - Petition on behalf of the one being tempted/persuaded to join the
16b - God will grant life to the one being tempted/persuaded to join the
16c - Don't bother praying for the antiChrists and those who've already
swallowed their poison.
Again, like 1 John 5:21, this statement of John's ("I do not say that one should pray for that") is not as strange as we might make it. In fact, in our plurastic (dare I say "conversant"?) ears, it probably comes across as quite harsh. But this is John's position and disposition toward those who were at one time with him, but now against him (1 John 2:19).
The dualism in John between light/darkness, Christ/antiChrist, life/death, love/does not love, God/devil, world/fellowship is absolutely stark. We're not used to that kind of stark language in our pluralistic (and Arminian) Christianity, especially when our family and friends are involved (as they were in Ephesus/Asia Minor). We're not used to speaking of excommunication with this kind of "demeanor".
There was no love lost between John and the antiChrists just as there was no love lost between the author of Hebrews and the apostasizers in Hebrews 10:26ff. John is applying Christ's judgment of John 8:24 to the antiChrists... we have to "feel" that kind of disposition in John when we read 5:16c. Certainly, this kind of disposition being applied to prayer and mediation isn't without precedent in the scriptures. Jeremiah 14:11, 1 Samuel 3:14, and Isaiah 22:14 are instances of the same. Christ himself reiterates Isaiah 22:14 (and probably 1 Samuel 3:14) in his pronouncement to the Pharisees in John 8:24 (and John 16:9), and further applies Jer. 14:11 in John 17:9. 1 John 5:16c is grounded in John 17:9, because John has already declared that these antiChrists and their followers have already "gone out into the world" (1 John 4:1,3-5; see also 1 John 2:15-19, 3:13, 5:19).
One thing we need to keep in mind about ALL of the passages cited in which there are limits placed on the *content* of our prayer (and -- limits on satisfaction for sin, i.e. 1 Sam. 3:14 and Isaiah 22:14) is that ALL involve apostasy. These "unbelievers" are not, to cop the Olds commercial, your father's "unbelievers". Throw in the woes pronounced on the Pharisees in Matthew, Christ's anger at the moneychangers, Christ's comment to Judas (it would have been better had he not been born), Paul's demeanor toward "housewreckers" in the Pastorals (1 Tim. 1: 19,20; 4:1, 5:8, 12, 15, 6:9; 2 Tim. 2:16-18, 3:2-9, 3:13, 4:3,4; Titus 1:10-16, 3:9, 10), the author of Hebrews' invocation of terrible Sinai and its judgment, and John's language in the Johannine corpus (1 John 4:1,3-5; see also 1 John 2:15-19, 3:13, 5:19, 2 John 7-11), and the comprehensive-biblical picture that is painted of apostasizers always places them in the position of beyond hope. That's what you have, IMHO, in 1 John 5:16c.