Ironically, despite the apostle's putative tendency for "contemptible speech," his discourse given in Athens (Acts 17) has been used to illustrate how rhetorical speeches should be given. See Lane Cooper, The Rhetoric of Aristotle (New York: Appleton-Century-Crofts, 1932), pages xxvii-xxix and see Cooper's discussion of Pauline enthymemes.
Paul is quite persuasive (though not necessarily oratorical in the strictest sense) and there does not seem to be anything contemptible about his speech at all in Acts 17. Examining 2 Cor. 10:10, we find that even those who opposed Paul in Corinth had to admit that his letters were weighty and forceful (ὅτι Αἱ ἐπιστολαὶ μέν, φησίν, βαρεῖαι καὶ ἰσχυραί) even if his speech was of no account (ὁ λόγος ἐξουθενημένος).
My research discloses it is possible that Paul was not a good looking fellow. His body might have been frail, his nose was possibly unsightly and his voice apparently was not pleasant to hear, some commentators say. Furthermore, according to Paul's own account, he evidently was not a trained rhetor (1 Cor. 1:17; 2:1-5).
His lack of training in rhetoric probably turned the stomachs of certain shallow individuals in Corinth. They were relying on human persuasion and liked to hear flowery speech (i.e., oratory) instead of putting their trust in God's power and listening to divine wisdom (1 Cor. 2:1-5). Therefore, it seems that Paul may have been an adequate speaker, after all, but he possibly was not formally trained in the art of rhetoric, although some historians believe that he did have a background in rhetoric and, from the sound of his letters and speech on Mars Hill, I am inclined to believe that the apostle knew something about rhetoric as well.
Nevertheless, like Augustine and other men who were rhetoricians before they started to profess belief in Christ, Paul may have decided to tone it down (so to speak) in order to avoid drawing attention to himself when he spoke publicly. This idea, I admit, is speculative. Yet I can hardly think of another way to account for Paul's expertise in employing literary or rhetorical devices in his writings: he certainly seems to have known something about rhetoric.
As a closing note, there is an interesting point that I found in James M. Scott's commentary on 2 Corinthians:
"There are striking parallels between this critique of
Paul and the critique of Moses. Moses was chosen and
made sufficient for his ministry in spite of his
'insufficiency,' which Exodus 4:10 links to his speech
defect (cf. S. Hafemann). In the same way, Paul can
assert his sufficiency in spite of his own personal
weakness, which, according to 2 Corinthians 10:10;
11:6, consist in part in his unimpressive speech.
Interestingly enough, Num. Rab. 18:9 attests to Moses'
wisdom and rhetorical ability. Josephus records that
Korah too was a capable speaker and very effective in
addressing a crowd (Ant. 4.14)" (2 Corinthians, pages