27 “Sitting against your brother you detracted”. And this “sitting” does belong to that whereof he has spoken above in, “has embraced.” For he that does anything while standing or passing along, does it not with pleasure: but if he for this purpose sits, how much leisure does he seek out to do it! That very evil detraction you were making with diligence, you were making sitting; you would thereon be wholly engaged; you were embracing your evil, you were kissing your craftiness.
“And against your mother's son you laid a stumbling-block.” Who is “mother's son”? Is it not brother? He would repeat then the same that he had said above, “your brother.” Hath he intimated that any distinction must be perceived by us? Evidently, brethren, I think a distinction must be made. Brother against brother does detract, for example's sake, as though for instance one strong, and now a doctor and scholar of some weight, does detract from his brother, one perchance that is teaching well and walking well: but another is weak, against him he lays a stumbling-block by detracting from the former.
For when the good are detracted from by those that seem to be of some weight and to be learned, the weak fall upon the stumbling-block, who as yet know not how to judge. Therefore this weak one is called “mother's son,” not yet father's, still needing milk, and hanging on the breast. He is borne as yet in the bosom of his mother the Church, he is not strong enough to draw near to the solid food of his Father's table, but from the mother's breast he draws sustenance, unskilled in judging, inasmuch as yet he is animal and carnal.
“For the spiritual man judges all things,” but “the animal man perceives not those things which are of the Spirit of God; for they are foolishness to him.” To such men says the Apostle, “I could not speak unto you as unto spiritual, but as unto carnal, as to babes in Christ I gave you milk to drink, not meat; for you were not able, but not even now are you able.” A mother I have been to you: as is said in another place, “I became a babe among you, even as a nurse cherishing her own children.” Not a nurse nursing children of others, but a nurse cherishing her own children.
For there are mothers who when they have borne give to nurses: they that have borne cherish not their children, because they have given them to be nursed; but those that cherish, cherish not their own, but those of others: but he himself had borne, he was himself cherishing, to no nurse did commit what he had borne; for he had said, “Of whom I travail again until Christ be formed in you.” He did cherish them, and gave milk. But there were some as it were learned and spiritual men who detracted from Paul.
“His letters indeed, say they, are weighty and powerful; but the presence of his body weak, and speech contemptible:” he says himself in his Epistle, that certain his detractors had said these words. They were sitting, and were detracting against their brother, and against that their mother's son, to be fed with milk, they were laying a stumbling-block. “And against your mother's son you laid a stumbling-block.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)