John Chrysostom — as excerpted in the Catena Aurea on Luke 9:18-22

“(Hom. 54. in Matt.) Timely also was our Lord’s command that no one should tell that He was Christ, in order that when offences should be taken away and the sufferings of the cross completed, a proper opinion of Him might be firmly rooted in the minds of the hearers. For that which has once taken root and afterwards been torn up. when fresh planted will scarcely ever be preserved. But that which when once planted continues undisturbed, grows up securely. For if Peter was offended merely by what he heard, what would be the feelings of those many who, after they had heard that He was the Son of God, saw Him crucified, and spit upon?”