5 But to those who are still babes in mind, and who as carnal, the apostle says, require to be nourished with milk, all discoursing on such a subject, wherein we deal not only with the believing, but also with the understanding and the knowing of what is spoken, must be burdensome, as being still unable to perceive such things, and be more fitted to oppress than to feed them. Whence it comes to pass that the spiritual, while not altogether silent on such subjects to the carnal, because of the Catholic faith which is to be preached to all, yet do not so handle them as, in their wish to simplify them to understandings that are still deficient in capacity, to bring their discourse on the truth into disrepute, rather than the truth that is in their discourse within the perceptions of their hearers.
Accordingly in his Epistle to the Colossians he says: “And though I be absent in the flesh, yet am I with you in the spirit, joying and beholding your order, and that which is lacking in your faith in Christ.” And in that to the Thessalonians: “Night and day,” he says, “praying more abundantly, that we might see your face, and might perfect that which is lacking in your faith.” Here we are, of course, to understand those who were under such primary catechetical instruction, as implied their nourishment with milk and not with strong meat; of the former of which there is mention made in the Epistle to the Hebrews of an abundant supply for such as nevertheless he would now have had to be feeding on solid food.
Accordingly he says: “Therefore leaving the word of the beginning of Christ, let us have regard to the completion; not laying again the foundation of repentance from dead works, and of faith toward God, of the doctrine of the baptismal font, and of the laying on of hands, and of resurrection of the dead, and of eternal judgment.” This is the copious supply of milk, without which even they cannot live, who have already indeed their reason sufficiently in use to enable them to believe, but who cannot distinguish good from evil, so as to be not only a matter of faith, but also of understanding (which belongs to the department of solid food). But when he includes doctrine also in his description of the milk, it is that which has been delivered to us in the Creed and the Lord's Prayer.
Source: Tractates on the Gospel of John (New Advent)