John Chrysostom — as excerpted in the Catena Aurea on Matthew 16:20-21

“For what having once had root has afterwards been torn up, if it is again planted, is with difficulty retained among the multitude; but what having been once rooted has continued ever after unmoved, is easily brought on to a further growth. He therefore dwells on these sorrowful things, and repeats His discourse upon them, that He may open the minds of His disciples.”