'Evidence' in the Bible

The one who believes in the Son of God [who adheres to, trusts in, and relies confidently on Him as Savior] has the testimony within himself [because he can speak authoritatively about Christ from his own personal experience]. The one who does not believe God [in this way] has made Him [out to be] a liar, because he has not believed in the evidence that God has given regarding His Son.

And the testimony is this: God has given us eternal life [we already possess it], and this life is in His Son [resulting in our spiritual completeness, and eternal companionship with Him].