‘He who believes on the Son of God has the witness in him. He who does not believe God has made him a liar, because he has not believed in the witness that God has borne concerning his Son. And the witness is this, that God gave to us eternal life, and this life is in his Son. He who has the Son has the life. He who does not have the Son of God does not have the life.'
So the one who believes on the Son of God has the witness in Him. He too was a witness. He witnessed to the truth while He was on earth. He witnessed to the truth after His resurrection. And He witnesses to the truth by imparting eternal life to His own and indwelling them. Thus the one who has believed on Him has received eternal life, and has the witness within himself. He has received the Spirit without Whom a man cannot be Christ's (Romans 8:9). He has received Jesus Christ Himself. On the other hand the one who does not believe God and accept His testimony concerning His Son, is making God a liar, is indeed making out that God is a liar like the Evil One, and this because he does not believe God's testimony concerning His Son.
For finally this is God's witness concerning His Son, that He has given us eternal life, and that that eternal life is only receivable through the Son. The one who has the Son has life, and the one who does not have the Son does not have life. It is as simple as that. Eternal life is found only through the Son. It cannot be received from the Father without the Son's participation.
So arriving at the end of the letter we come back to the beginning. The Word of life has come (1 John 1:1) and His commandment is eternal life (John 12:50 with 1 John 2:7). This is central to everything. It is the Gospel. All must be born from above (John 3:6). And this life is received by those who believe, not on a human being, not on a phantasm, but on God's true Son, Who came as a human being, died on a cross providing cleansing (1 John 1:7) and propitiation for sin (1 John 2:2) and rose again from the dead. Such people then have the witness in themselves, the witness of the anointing, of the Holy Spirit and the word, and the witness of the life within them.
This life is not available to the false teachers. They deny Jesus His Sonship. They reject God's full testimony concerning His Son. They make Him a liar. For them there is no means of propitiation. For them there is no life. For it is they who are liars, and preach lies. They believe ‘the Lie' (2 Thessalonians 2:11). God's testimony to His Son lies in the fact that He demonstrated His lifegiving power by raising Him from the dead as the Son by the Holy Spirit, and enabling Him thereby to give life to those who believed in Him.
And those who do believe on Him receive life. This lifegiving power is in the Son, so that he who has the Son has life, and he who does not have the Son does not have life. Thus those have the Son who believe in Him and have been given life by God through His Spirit. Those who have not believed in the Son do not have life.