Belief in your heart comes before confession with your mouth. What you believe in your heart, you must confess with your mouth. The gospel is gospel not when it is believed but when it is confessed.
The rock on which Jesus built the church is not Peter’s confession but a confessing Peter.
Christian faith is not a private deal you make with God. It is a public confession. Christianity is both a belief and a confession. It is not enough that God knows you’re a Christian. Others must know it too.