If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. And this is the record, that God hath given to us eternal life, and this life is in his Son. He that hath the Son hath life; and he that hath not the Son of God hath not life. (1 John 5:9-12 KJV)
John argues from the lesser to the greater. We believe the witness of men, how much more should we believe the witness of God?!
What is the witness of God?
The witness of God is that he that believes on the Son of God has the witness in himself...that God has given us eternal life, and this life is in His Son. He that has the Son has life; and he that has not the Son of God has not life.
The Spirit itself beareth witness with our spirit, that we are the children of God: (Romans 8:16 KJV)