Become as little children

But truly Jesus set Himself in the midst of them, a little one who had come not to be ministered to, but to minister, that He might be a pattern of holiness. Others interpret the little one of the Holy Spirit, whom He set in the hearts of His disciples, to change their pride into humility. And he said, "I say to you, Except you be converted, and become as little children, you shall not enter into the kingdom of heaven." He does not enjoin on the Apostles the age, but the innocence of infants, which they have by virtue of their years, but to which these might attain by striving; that they should be children in malice, not in understanding.

As though He had said, "As this child, whom I set before you as a pattern, is not obstinate in anger, when injured does not bear it in mind, has no emotion at the sight of a fair woman, does not think one thing while he speaks another; so you, unless you have the like innocence and purity of mind, shall not be able to enter into the kingdom of heaven."

References:
Commentariorum In Evangelium Matthaei (St. Jerome)
Morals on the Book of Job, Book 6 (St. Gregory the Great)