Ah, 1 John 5:18... Well, we shouldn't cherry-pick, right, Robert? If you take it in the context of 1 John 5 (not hard, since it's the same chapter, and only a total of 21 verses at that), and even just in the context of the immediately preceding verses (16 and 17), which say:
"If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life ~ to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. All wrongdoing is sin, but there is sin that does not lead to death."
So John is actually
assuming (and implicitly declaring) that brothers will indeed sin, because, as he said earlier, "...if we say we are without sin, we deceive ourselves and the truth is not in us" (1 John 1:8). Sin not leading to death is sin for which forgiveness is possible, because 1.) forgiveness is sought, and 2.) God is willing to grant it. On the other hand, "sin that leads to death" is sin that is 1.) unrepented of and 2.) of the kind or nature that John has warned about
throughout the letter ~ resolute rejection of the true doctrine about Christ, chronic disobedience to God's commandments, persistent lack of love for fellow believers ~ all indications of a lack of saving faith ~ which will not be forgiven.
And then comes verse 18, which says:
"We know that everyone who has been born of God does not keep on sinning, but he who was born of God protects him, and the evil one does not touch him."
So, yet another passage that you are horribly mistaken on, because of your taking this verse and that verse in isolation from other parts of the Bible and even adjacent verses. Again, you're railing against antinomianism, and no "Calvinist OSAS" is guilty of that.
Grace and peace to you, Robert.