The phrase appears four times in Paul’s epistles:
But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man) (Rom 3:5)
I speak after the manner of men because of the infirmity of your flesh: for as ye have yielded your members servants to uncleanness and to iniquity unto iniquity; even so now yield your members servants to righteousness unto holiness. (Rom 6:19)
Brethren, I speak after the manner of men; Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto. (Gal 3:15)
Say I these things as a man? or saith not the law the same also? (1 Cor 9:8)
Commenting on the meaning of the phrase, David Daube wrote:
The phrase is an apology for a bold statement which, without such an apology, might be considered near-blasphemous—be it because it is too anthropomorphic or be it because it sounds otherwise lacking in reverence for God or an established religious idea.
Let us look at the four texts. In Romans 3.5 Paul apologizes for framing the hypothesis that God may be unrighteous. Of course he refutes the hypothesis. But the mere fact that he puts it, unless accompanied by this apology, might be judged blasphemous by a Jewish public.
In Romans 6.19 he apologizes for admonishing the recipients of the letter to become ‘slaves to righteousness’. [The Old Testament and Rabbinic literature] never describe the faithful as slaves of anyone or anything but God. Paul’s terms is not only, as Dodd remarks, ‘sub-Christian’, it is above all ‘sub-Jewish’. He has recourse to it in order to impress on antinomian doctrinaires that, just as formerly they were eager to sin, so now they should be eager to do good, to live saintly lives. Nevertheless the apology is needed since, without it, in the eyes of good Jews, his exhortation would be objectionable.
In Galatians 3.15 he compares a disposition made by God with one made by a man, and besides he considers the question whether it is subject to repeal or change. However vigorously he denies this possibility, that mere hypothesis requires an apology. The case is very similar to Romans 3.5.
I have left 1 Corinthians 9.8 to the last, the matter being a little more complicated. But even here there is no doubt as to the meaning of the phrase.
Paul contends that an apostle is entitled to support from his congregations, as a shepherd takes milk from his flock; and he adds that he does not intend to apologize, i.e. he is not ‘speaking after the manner of men’. Obviously he assumes that some of the recipients of the letter will expect this apology here. They will expect him to indicate that his words are not to be taken literally, that they are a bold metaphor which, without the apology, might be deemed wanting in reverence for God or an accepted religious custom.
Now in New Testament times the Rabbis were agreed that the word of God must be taught gratuitously. Hillel (about 30 B.C.) said ‘He who makes use of the crown (the Torah) will perish’, and Zadok (about A.D. 50), ‘Make not the words of the Torah a spade wherewith to dig’ (Mish. Ab. 1.13, 4.5). A few conclusions were made in the case of elementary instruction of children; but even they seem to have been introduced at a later date. It is, then, the Jewish traditionalists among his addressees to whom Paul declares that there is to be no apology to mitigate his assertion. This assertion, that apostles are entitled to support, must be taken literally, and if it causes offence it cannot be helped.
That he anticipates opposition of the king we have outlined is confirmed by the manner he argues. Having made his assertion, he does not simply refer to the pronouncement by which Jesus authorized his messengers to live by their office. On the contrary, he begins by adducing a passage from the Law, from Deuteronomy—‘Thou shalt not muzzle the mouth of the ox that treadeth out the corn’—and this he interprets most freely with a view to extracting from it the proof he needs. But even when he does on to Jesus’s pronouncement, he represents it as a mere extension by analogy of Old Testament regulations: ‘They which minister about holy things live of the temple—even so hath the Lord ordained that they which preach the gospel should live of it.’ Neither in Matthew now in Luke (Matt. 10.10, Luke 10.7) is Jesus’s pronouncement rested on Old Testament authority (though, significantly, in Matthew it appears in a far more restrained version than in Luke: Matthew mentions a right to meat only, Luke one to hire). 1 Timothy also (5.18), though like 1 Corinthians it quotes first the Deuteronomic law—‘Thou shalt not muzzle’ and so on—and only then Jesus’s pronouncement, does not connect the latter with Old Testament rules.
Paul in 1 Corinthians has to deal with the charge that the right he claims for apostles is contrary to a hallowed religious principle. Despite the charge he insists on the right, explaining that he does not here ‘speak after the manner of men’. (David Daube, The New Testament and Rabbinic Judaism [University of London, 1956; repr., Peabody, Mass.: Hendrickson Publishers, Inc., 1998], 394-96, comment in square bracket added for clarification)