1 John 5:10, 20

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 we [have seen and] know [by personal experience] that the Son of God has [actually] come [to this world], and has given us understanding and insight so that we may [progressively and personally] know Him who is true; and we are in Him who is true—in His Son Jesus Christ. This is the true God and eternal life.