Jesus Christ is the one who came with the water of his baptism and the blood of his death. He came not only with the water, but with both the water and the blood. And the Spirit himself testifies that this is true, because the Spirit is truth.
So those who believe in the Son of God have this testimony in their own heart; but those who do not believe God, have made a liar of him, because they have not believed what God has said about his Son.
If you see a believer commit a sin that does not lead to death, you should pray to God, who will give that person life. This applies to those whose sins do not lead to death. But there is sin which leads to death, and I do not say that you should pray to God about that.
We know that the Son of God has come and has given us understanding, so that we know the true God. We live in union with the true God---in union with his Son Jesus Christ. This is the true God, and this is eternal life.