1John 5:14,15 This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us. And if we know that he hears us— whatever we ask— we know that we have what we asked of him.

John mentioned something similar to this in 1John 3:21,22 saying, "if our hearts do not condemn us, we have confidence before God and receive from him anything we ask, because we obey his commands and do what pleases him." And I'll reiterate my comment there that promises like Matt 21:22 "If you believe, you will receive whatever you ask for in prayer." given under the assumption that we are obeying his commands and doing what pleases him, as you can also see from this verse where the promise of answered prayer is contingent upon us praying in accordance with his will.

But satisfying that contingency we can have confidence that God hears us and will take action on our requests. What is also implied is that if you don't pray, God may not take such action.

1John 5:16,17 If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. All wrongdoing is sin, and there is sin that does not lead to death.

Again, continuing to keep a consistent hermeneutic, invoking the significance of the present tense in the Greek we note that as "commit a sin" is in the present tense, John is referring to a sin which is characteristic of this "brother's" lifestyle. But as John had said a number of times previously in this letter that "no one who is born of God will continue to sin" (1John 3:9), and as he does just 2 verses later, "We have perceived that whoever has been born of God does not characteristically sin;" (1John 5:18), I take it that this man is a brother in name only.

And this makes sense because it speaks of praying for this person so that God may give him life - indicating that he didn't have life. And as we just read in this chapter "He who has the Son has life; he who does not have the Son of God does not have life." 1John 5:12, apparently such a "brother" is viewed as not having the Son. This is a prayer for a "nominal Christian". That is, a Christian who is one in name only.

As the letter of 1John is all about identifying nominal Christians, here John is telling us what to do if we uncover one. Pray for that person that God may give them life.

The sin unto death is a condition in which the "Christian" so strongly resists correction as to attribute the working of the Holy Spirit to the devil. That is, "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace." Heb 6:4,6 In fact "It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. Of them the proverbs are true: A dog returns to its vomit, and, A sow that is washed goes back to her wallowing in the mud." 2Peter 2:21-22

Jesus said, "Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come." Mt 12:32 And why is that? Because compliance to the leading of the Holy Spirit is necessary to coming to saving faith. Rejecting such leading with strong conviction cut's oneself off from the necessary path to salvation. It's impossible to bring such people back to repentance.

Nonetheless John is not forbidding prayers for such people, for as it is it may be difficult to determine whether a person had sinned in this manner, but if a person apostacizes it is not required of the Christian to pray for such people.

1John 5:18,19  We know that anyone born of God does not continue to sin; the one who was born of God keeps him safe, and the evil one cannot harm him.  We know that we are children of God, and that the whole world is under the control of the evil one.

John reiterates the fact that those born of God do not continue to sin, just as he said in 1John 3:9. And bringing this up here supports the idea that the previous two verses were referring to nominal Christians - those who continue to sin.

And as we also read in 1John 3:9 the reason why those born of God don't continue to sin has to do with the nature of the rebirth. "No one who is born of God will continue to sin, because God’s seed remains in him; he cannot go on sinning, because he has been born of God." John alludes to this idea here in the phrase "the one who was born of God keeps him safe, and the evil one cannot harm him.", though there are nuances of interpretation.
 

For example, "the one who was born of God" could refer to Jesus since "was born" is in the aorist indicating an historic past event as opposed to the perfect tense which John uses earlier in the verse to referring to "anyone born of God". And Jesus said, "While I was with them, I protected them and kept them safe by that name you gave me." John 17:12 and "My prayer is not that you take them out of the world but that you protect them from the evil one." John 17:15, not to mention also the analogy he makes in John 10 of himself being a shepherd taking care of his sheep, and in particular John 10:28 "I give them eternal life, and they shall never perish; no one can snatch them out of my hand." (well I guess I mentioned it anyway!) And this is the most likely interpretation based upon the Greek manuscripts upon which many modern translations, like the NASB and NIV are based.

But there is a textual variant in some manuscripts, which is carried over in the KJV where the word "him" in "keeps him" is actually "himself", which means this could be read that those born of God keep (take care of) themselves such that they don't continue to sin.

In fact in it could be argued that both of these are the case, kind of like Paul said, "Therefore, my dear friends, as you have always obeyed— not only in my presence, but now much more in my absence— continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act according to his good purpose." Php 2:12,13

1John 5:20,21 We know also that the Son of God has come and has given us understanding, so that we may know him who is true. And we are in him who is true— even in his Son Jesus Christ. He is the true God and eternal life. Dear children, keep yourselves from idols.

Jesus said, "this is eternal life: that they may know you, the only true God, and Jesus Christ, whom you have sent."John 17:3 Eternal life is all about knowing God and knowing Jesus Christ the Son of God. And we can only know God through the Son, as it is written, "No one has ever seen God, but God the One and Only, who is at the Father’s side, has made him known." John 1:18 He alone is the true God and eternal life. Therefore keep yourself from imitations.