1Whoever believes that Jesus is the Christ has been born of God. Whoever loves the Father also loves the child who is born of him. 2By this we know that we love the children of God, when we love God and keep his commandments. 3For this is loving God, that we keep his commandments. His commandments are not grievous. 4For whatever is born of God overcomes the world. This is the victory that has overcome the world: your faith. 5Who is he who overcomes the world, but he who believes that Jesus is the Son of God?
6This is he who came by water and blood, Jesus Christ; not with the water only, but with the water and the blood. It is the Spirit who testifies, because the Spirit is the truth. 7For there are three who testify: 8the Spirit, the water, and the blood; and the three agree as one. 9If we receive the witness of men, the witness of God is greater; for this is God’s testimony which he has testified concerning his Son. 10He who believes in the Son of God has the testimony in himself. He who doesn’t believe God has made him a liar, because he has not believed in the testimony that God has given concerning his Son. 11The testimony is this: that God gave to us eternal life, and this life is in his Son. 12He who has the Son has the life. He who doesn’t have God’s Son doesn’t have the life.
13These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.
14This is the boldness which we have toward him, that if we ask anything according to his will, he listens to us. 15And if we know that he listens to us, whatever we ask, we know that we have the petitions which we have asked of him.
16If anyone sees his brother sinning a sin not leading to death, he shall ask, and God will give him life for those who sin not leading to death. There is sin leading to death. I don’t say that he should make a request concerning this. 17All unrighteousness is sin, and there is sin not leading to death.
18We know that whoever is born of God doesn’t sin, but he who was born of God keeps himself, and the evil one doesn’t touch him. 19We know that we are of God, and the whole world lies in the power of the evil one. 20We know that the Son of God has come and has given us an understanding, that we know him who is true; and we are in him who is true, in his Son Jesus Christ. This is the true God and eternal life.
21Little children, keep yourselves from idols.
6 This is he who came by water and blood, Jesus Christ; not with the water only, but with the water and the blood. It is the Spirit who testifies, because the Spirit is the truth. 7 For there are three who testify: 8 the Spirit, the water, and the blood; and the three agree as one. 9 If we receive the witness of men, the witness of God is greater; for this is God’s testimony which he has testified concerning his Son.
We see in the verses above a discussion dealing with Spirit, water, and blood. Again, we must remember to whom John was responding. He was responding to the gnostics, some of whom denied Jesus' humanity. For two thousand years, believers have discussed what these verses might mean. Some Gnostics believed that Jesus was born a human and, upon baptism, "transitioned" into deity and humanity. Then they thought that He "gave up His Spirit" and became a man solely again on the cross. This is a distortion and doesn't follow scripture. It is men trying to overthink what the Bible tells us. John recognized that Jesus came by "water and blood." In other words, He was God in human flesh, died on the cross, and remained fully God and fully man. We would do well to focus on the plain meaning of Scripture.
How do the Spirit, the water, and the blood testify? We find the key to understanding this passage in verse 7, where we read, "Now there are these three that testify." There are many excellent commentaries that explain this, including Dr. Thomas Constable's notes [1], which are freely available online. I also often refer to the Believer's Bible Commentary. These are in agreement on the witness of Jesus being: 1) Through the water at His baptism where the Spirit descended and the Father spoke, 2) that same baptism where the Spirit appeared, testified to the divinity of Christ, and 3) His blood on the cross testified as to His humanity. The simplest explanation I got was from my wife's teaching notes of 20 years of Bible studies. She is a skilled Bible student and teacher.
I quote her, "John's reference can be summarized as follows: 'born of water, he was baptized, born of the spirit (evidenced by the Holy Spirit at His baptism), He was divine. Born of blood, He was fully human." These kinds of passages seem cryptic and are easily misunderstood unless a faithful student understands the audience and the context in which John spoke. For you and me, it helps us to remember that Jesus was God. He was who He said that He was, and He will do what He says He will do. And reader, that is very comforting.
[1]https://planobiblechapel.org/tcon/notes/pdf/1john.pdf (see pages 108-110)