1 “Children, it is the last hour.” In this lesson he addresses the children that they may make haste to grow, because “it is the last hour.” Age or stature of the body is not at one's own will. A man does not grow in respect of the flesh when he will, any more than he is born when he will: but where the being born rests with the will, the growth also rests with the will. No man is “born of water and the Spirit,” except he be willing. Consequently if he will, he grows or makes increase: if he will, he decreases.
What is it to grow? To go onward by proficiency. What is it to decrease? To go backward by deficiency. Whoso knows that he is born, let him hear that he is an infant; let him eagerly cling to the breasts of his mother, and he grows apace. Now his mother is the Church; and her breasts are the two Testaments of the Divine Scriptures. Hence let him suck the milk of all the things that as signs of spiritual truths were done in time for our eternal salvation, that being nourished and strengthened, he may attain to the eating of solid meat, which is, “In the beginning was the Word, and the Word was with God, and the Word was God.” Our milk is Christ in His humility; our meat, the selfsame Christ equal with the Father.
With milk He nourishes you, that He may feed you with bread: for with the heart spiritually to touch Christ is to know that He is equal with the Father.
Source: Homilies on the First Epistle of John (New Advent)