The Greater Witness
1 John 5:9 If we receive the witness of men, the witness of God is greater; for this is the witness of God which He has testified of His Son.
10 He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son.
11 And this is the testimony: that God has given us eternal life, and this life is in His Son.
12 He who has the Son has life; he who does not have the Son of God does not have life.
Simply, if you have recieved Jesus, you have abundant life! This life only comes through the Son. To not believe Jesus is to call God a liar, in which Im not comfortable doing, like, at all! God the Father testifies that the Son is fully God. With God, It is impossible for Him to testify anything other than Jesus is God.