6 But let us be far from supposing that there is any contrariety between this milk and the food of spiritual things that has to be received by the sound understanding, and which was wanting to the Colossians and Thessalonians, and had still to be supplied. For the supply of the deficiency implies no disapproval of that which existed. For even in the very food that we take, so far is there from being any contrariety between milk and solid food, that the latter itself becomes milk, in order to make it suitable to babes, whom it reaches through the medium of the mother's or the nurse's body; so did also mother Wisdom herself, who is solid food in the lofty sphere of angels, condescend in a manner to become milk for babes, when the Word became flesh, and dwelt among us. But the man Christ Himself, who in His true flesh, true cross, true death, and true resurrection is called the pure milk of babes, is, when rightly understood by the spiritual, found to be the Lord of angels.
Accordingly, babes are not to be so fed with milk as always to remain without understanding the Godhead of Christ; nor are they to be so withdrawn from milk as to turn their backs on His manhood. And the same thing may also be stated in another way in this manner: they are neither so to be fed with milk as never to understand Christ as Creator, nor so to be withdrawn from milk as ever to turn their backs on Christ as Mediator. In this respect, indeed, the similitude of maternal milk and solid food scarcely harmonizes with the reality as thus stated, but rather that of a foundation: for when the child is weaned, so as to be withdrawn from the nourishment of infancy, he never looks again among solid food for the breasts which he sucked; but Christ crucified is both milk to sucklings and meat to the more advanced.
And the similitude of a foundation is on this account the more suitable, because, for the completion of the structure, the building is added without the foundation being withdrawn.
Source: Tractates on the Gospel of John (New Advent)