What does Paul mean when he says 'faith in Christ'?

Paul means trusting in the finished work of redemption—that Christ died, was buried, and rose again—not merely believing He was the Messiah, which belonged to Peter's domain. This faith in Christ Jesus, plus nothing, makes one a child of God, apart from Law-keeping.

Les explains that in Galatians 3, Paul contrasts Law-keeping with faith. The Law has been satisfied and completely paid in full through the work of the Cross; it has done its job and is off the scene. Therefore we enter in by faith plus nothing. Many so-called Christians try to add works, but this verse excludes that. Paul never speaks of just believing Jesus was the Messiah—that was Peter's domain. When Paul speaks of faith in Christ, he means the finished work of redemption: Christ died, was buried, and rose again. By that alone we become the children of God.

What did Paul mean by faith in Christ rather than just believing he was the Messiah?

Paul meant trusting in the finished work of redemption: that Christ died, was buried, and rose again. It is faith plus nothing, because the Law was satisfied and paid in full at the Cross. Believing Jesus was the Messiah belonged to Peter’s domain, not Paul’s phrase “faith in Christ.”