As impious as his whole argumentative, "I know everything" approach seems, I suppose many canonized saints dealt the same way with heretics in their writings, being very uncharitable at times.
The guy has a point in that this verse really does seem to indicate we are to have some surety in our salvation, which definitely is not something taught in Orthodox circles. It's usually more "I'm the worst sinner in the world. Please don't throw me into Hell forever!"
Anybody care to actually respond to the criticism?
St. John (I)
5:1 Every one who believes that Jesus is the Christ is a child of God, and every one who loves the parent loves the child. 2 By this we know that we love the children of God, when we love God and obey his commandments.
3 For this is the love of God, that we keep his commandments. And his commandments are not burdensome. 4 For whatever is born of God overcomes the world; and this is the victory that overcomes the world, our faith
. 5 Who is it that overcomes the world but he who believes that Jesus is the Son of God? 6 This is he who came by water and blood, Jesus Christ, not with the water only but with the water and the blood. 7 And the Spirit is the witness, because the Spirit is the truth. 8 There are three witnesses, the Spirit, the water, and the blood; and these three agree.
14 And this is the confidence which we have in him, that if we ask anything according to his will he hears us. 15 And if we know that he hears us in whatever we ask, we know that we have obtained the requests made of him. 16 If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. 17 All wrongdoing is sin, but there is sin which is not mortal. 18 We know that any one born of God does not sin, but He who was born of God keeps him, and the evil one does not touch him
. 19 We know that we are of God, and the whole world is in the power of the evil one
1:1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon and touched with our hands, concerning the word of life-- 2 the life was made manifest, and we saw it, and testify to it, and proclaim to you the eternal life which was with the Father and was made manifest to us-- 3 that which we have seen and heard we proclaim also to you, so that you may have fellowship with us; and our fellowship is with the Father and with his Son Jesus Christ. 4 And we are writing this that our joy may be complete. 5 This is the message we have heard from him and proclaim to you, that God is light and in him is no darkness at all. 6 If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth
; 7 but if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. 8 If we say we have no sin, we deceive ourselves, and the truth is not in us.
9 If we confess our sins, he is faithful and just, and will forgive our sins and cleanse us from all unrighteousness.
10 If we say we have not sinned, we make him a liar, and his word is not in us
If it were as the rant of the purported apostate misconstrues St. John, the saved would not sin. But St. John tells the saved that we must confess our sins, otherwise the saving Truth is not in us. Hence the rant's misconstruction cannot be right.