1 John 5:10, 20

He that believeth on the Son of God hath the witness in him: he that believeth not God hath made him a liar; because he hath not believed in the witness that God hath borne concerning his Son.

And we know that the Son of God is come, and hath given us an understanding, that we know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.