As a believer we are witnesses by the change in our lives, (as well as others who knew us-as an old creation) coming out of darkness and into the marvelous light of Christ. The witness of the change in our lives should always point to the witness of Jesus for their lives also. 

And even though we can be a witness for Him, and even though people may not believe our witness of Him, rejecting Him only keeps those who do, from receiving His gift of Abundant Life. Believing in Him or not doesn't change who He is or what he's done. As 2 Timothy 2:13 notes, "If we are faithless, He remains faithful. He can't deny Himself."

Jesus has a greater Witness than man who validates Him, and Jesus also validates the only True God. (John 17:3)

Scripture Ref.

John 5:37-47; The Witness
Deuteronomy 32:30,
Matthews 18:30,
Philippians 2:6,