This is the one who came by water and blood--Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth.
Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son.
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.
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.