Luke 9:47

And Jesus, perceiving the thought of their heart, took a child, and set him by him,

Jesus discerned the unspoken reasoning in the disciples' hearts even while they attempted to conceal it. What He saw hidden there was their quarrel over which of them would rank as the greatest. To answer it He took a child and placed him by His side in the midst of the disciples as He made ready to instruct them.

The way He handled the child displayed His tenderness. The point He pressed upon them was that they themselves must become like that child. That childlike humility was not optional, for only by becoming humble as a child could they enter the kingdom of heaven. Peter came to grasp this teaching more deeply afterward when the contrast between his own weakness and Christ's power dawned upon him, and that grasp fitted him to trust Jesus, to obey Him, and to follow Him.