The Testimony of God; 1 John 5:9–13
1 John 5:9 NASB "If we receive the testimony [witness] of men, the testimony [witness] of God is greater …" Here John uses what is called an a fortiori argument. This is a Latin term for a certain type of logic. Then logic means literally from the stronger or from the greater. For example, if you can sit down and eat and entire fried chicken in one sitting then I could say you would also be able to eat a drumstick. If you can do something much harder and more difficult then obviously you can do something that is less difficult. Here John is arguing from the lesser to the greater that if you believe men and what they say you should also believe God, for God is greater than men. So if you receive the legal testimony of those who were eye-witnesses of Jesus' life, His death burial and resurrection, then the witness of God is even greater. The tendency for all of us is to put too much weight on the witness of men and to human experiences. "…for the testimony of God is this, that He has testified concerning His Son.
1 John 5:10 NASB "The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given concerning His Son." If we trust in Jesus Christ as out Saviour then this witness becomes internal. We have accepted it as true and that, of course, is what produces regeneration: He who believes in Jesus as the Son of God; not just in the Man but in Jesus as full deity. The phrase "Son of God" indicates full deity. If a person does not believe God then John says this has made God a liar. He says the same kind of thing in 1 John 1:10 when he says that if we say we have not sinned we make Him a liar. Jesus claims to be the exclusive way to God—John 14:6. Well he either is, in which case that is true, or he is not telling the truth. If He is not telling the truth He is a liar. If He is a liar then He cannot be a good man, a religious innovator or a great moral teacher; He is basically a deceiver, and there have been millions and millions of people down through the ages who trusted Him and He is not worth trusting. So we can't have a legitimate option of thinking that he is a good man. He is either a liar or He is crazy, because anyone who says I am the only way to God and I am God is either telling the truth or he is self-deceived. If they are not intentionally deceiving people as a liar then they are self-deceived, and if they are self-deceived and they think they are God then they are nuts! The only options are that He is a liar, He is crazy, or He is the Lord of the universe as he claimed to be. He is the Son of God and if we say He wasn't then what we are saying is that God is a liar and God has deceived us in the Scripture. That is pure blasphemy. God's testimony is through the apostles and through their apostolic witness as contained in the Gospels.
1 John 5:11 NASB "And the testimony is this, that God has given us eternal life, and this life is in His Son." In 1 John 1:2 John said: "and the life was manifested, and we have seen and testify and proclaim to you the eternal life, which was with the Father and was manifested to us," that is, Jesus Christ is equal to eternal life. They are declaring Him as the one who is equivalent to life. Then in 1 John 3:15 we notice that there is another dimension to eternal life. There is one dimension where we receive it at salvation and we will live eternally but there is another sense of eternal life and that has to do with the fullness, the full dimension of our spiritual life that we are enjoying in this life. "Everyone who hates his brother is a murderer; and you know that no murderer has eternal life abiding in him." That is not talking about salvation, about someone who is not a believer, because if you hate your brother that means that both you and the other person are brothers in Christ; you are both saved. "…no murderer has eternal life abiding in him." He is a believer and he has eternal life but he is not enjoying it and it is not abiding in him. Jesus said: "The thief comes only to steal and kill and destroy; I came that they may have life, and have {it} abundantly." That is the difference between the reality of eternal existence and the full enjoyment and possession of that life here in time. So when John is talking here at the end he is talking not so much about simply the reception of non-ending life in heaven but that the full expression and dimension of that in this life is the result of abiding in Christ.
1 John 5:12 NASB "He who has the Son has the life; he who does not have the Son of God does not have the life." This is the key statement. He who has the Son has life. The concept there of having is the Greek verb echo [e)xw] in the present tense which emphasises a present possession.
1 John 5:13 NASB "These things I have written [referring to what he has just now stated] to you who believe in the name of the Son of God, so that you may know that you have eternal life." There are two things here. First that you may know that you have eternal life. You can have eternal life and not know it because you are a doubting Thomas, because you are doubting God's ability; but also that you might continue to believe in the name of the Son of God. That is crucial. That was the problem in this church, that the false teachers were coming in and were saying that Jesus really isn't the Son of God, and so these false teachers no longer believed that Jesus was the Son of God. They were not continuing to believe in the name of the Son of God. John is warning them that if they want to have full possession of eternal life here in time then they have to continue to rely upon Jesus as He is, i.e. in His full deity and as the Son of God.