Mark 10:14

Wherefore He says not, for of these, but of such is the kingdom of God, that is, of persons who have both in their intention and their work the harmlessness and simplicity which children have by nature. For a child does not hate, does nothing of evil intent, nor though beaten does he quit his mother; and though she clothe him in vile garments, prefers them to kingly apparel; in like manner he, who lives according to the good ways of his mother the Church, honours nothing before her, nay, not pleasure, which is the queen of many; wherefore also the Lord subjoins, Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.

(Matt. tom. xv. 7) If any of those who profess to hold the office of teaching in the Church should see a person bringing to them some of the foolish of this world, and low born, and weak, who for this reason are called children and infants, let him not forbid the man who offers such an one to the Saviour, as though he were acting without judgment. After this He exhorts those of His disciples who are already grown to full stature to condescend to be useful to children, that they may become to children as children, that they may gain children; for He Himself, when He was in the form of God, humbled Himself, and became a child. On which He adds: For of such is the kingdom of heaven. (1 Cor. 9:22)