Secara singkat
Matthew 16:15’s question was for all the disciples. In Matthew 16:16, Peter answers on behalf of all of them. They could not all respond simultaneously, so Peter acts here as primus inter pares (first among equals), and when Jesus responds to this confession in Matthew 16:20, he addresses all of them.
The question of Matthew 16:15 was expressly posed to all the disciples. Peter’s answer in Matthew 16:16 therefore ought to be understood as an answer on behalf of all of them. They could not all respond simultaneously and as such Peter therefore acts here as primus inter pares (first among equals). When Jesus later responds to this confession in Matthew 16:20, he also addresses the group of disciples as a whole.1
16 Simon Peter replied, “You are the Christ, the Son of the living God.”