Acts 5:32 (NIV)
We are witnesses of these things.
A man may not believe in the God-Man Jesus the Christ, but they may believe in a fellow man. None of the Apostles were perfect in God's sight. Paul was a murderer and blasphemer, and Peter was a betrayer, while Thomas was an agnostic. But God chose the lowly things of this world to do His will and bidding. A man may not believe in the perfect Man, Jesus of Nazareth. But they may believe fellow reprobates like him who were in darkness but were brought into the glorious light of the risen Christ.