3 Having ascertained this, therefore, at the outset, that the very things, which are equally heard by the spiritual and the carnal, are received by each according to the slender measure of his own capacity—by some as babes, by others as those of riper years—by one as milk nourishment, by another as solid food—there seems no necessity for any matters of doctrine being retained in silence as secrets, and concealed from infant believers, as things to be spoken of apart to those who are older, or possessed of a riper understanding; and let us regard it as needful to act thus, just because of the words of the apostle, “I could not speak unto you as unto spiritual, but as unto carnal.”
For even this very statement of his, that he knew nothing among them but Jesus Christ and Him crucified, he could not speak unto them as unto spiritual, but as unto carnal; because even that they were not able to receive as spiritual. But all who were spiritual among them received with spiritual understanding the very same truths which the others only heard as carnal; and in this way may we understand the words, “I could not speak unto you as unto spiritual, but as unto carnal,” as if he said, What I did speak, you could not receive as spiritual, but as carnal.
For “the natural man”— that is, the man whose wisdom is of a mere human kind, and is called natural [literally, soulish] from the soul, and carnal from the flesh, because the complete man consists of soul and flesh— “perceives not the things of the Spirit of God;” that is, the measure of grace bestowed on believers by the cross of Christ, and thinks that all that is effected by that cross is to provide us with an example for our imitation in contending even to death for the truth.
For if men of this type, who have no desire to be anything else than men, knew how it is that Christ crucified is “made of God unto us wisdom, and righteousness, and sanctification, and redemption, that, according as it is written, He that glories, let him glory in the Lord,” they would doubtless no longer glory in man, nor say in a carnal spirit, “I am of Paul, and I of Apollos, and I of Cephas;” but in a spiritual way, “I am of Christ.”
Source: Tractates on the Gospel of John (New Advent)