Either Jesus is a liar and a lunatic, or He is the real thing - the Son of the living God, the crucified Christ, and the risen Savior of the world. Other options are not available. Peter confesses Jesus to be the Christ. To confess is to say back what you have heard, to say with your lips what your heart believes and what your ears have heard. Peter speaks when the other disciples are silent.
Silence is a sneaky form of denial.
When you don't have anything good to say, you say nothing at all. To keep silent when confession is called for is unbelief. The devil, the world, our own sinful nature in us prefer silence - the silence of sins unacknowledged, the silence of Christ unconfessed. If faith is created and fed by hearing the Word of Christ, then silence is faith's worst enemy, and confession is faith's greatest ally. Peter confesses Jesus as the Christ and is blest in his confession.