For his letters - The letters which he has sent to the church when absent. Reference is had here probably to the First Epistle to the Corinthians. They might also have seen some of Paul's other epistles, and been so well acquainted with them as to he able to make the general remark that he had the power of writing in an authoritative and impressive manner.
Say they - Margin, "Said he." Greek (φησὶν phēsin) in the singular. This seems to have referred to some one person who had uttered the words - perhaps some one who was the principal leader of the faction opposed to Paul.
Are weighty and powerful - Tyndale renders this: "Sore and strong." The Greek is, "heavy and strong" (βαρεῖαι καὶ ἰσχυραί bareiai kai ischurai. The sense is, that his letters were energetic and powerful. They abounded with strong argument, manly appeals, and impressive reproof. This even his enemies were compelled to admit, and this no one can deny who ever read them. Paul's letters comprise a considerable portion of the New Testament; and some of the most important doctrines of the New Testament are those which are advocated and enforced by him; and his letters have done more to give shape to the theological doctrines of the Christian world than any other cause whatever. He wrote 14 epistles to churches and individuals on various occasions and on a great variety of topics; and his letters soon rose into very high repute among even the inspired ministers of the New Testament (see 2-Peter 3:15, 2-Peter 3:16), and were regarded as inculcating the most important doctrines of religion. The general characteristics of Paul's letters are:
(1) They are strongly argumentative. See especially the Epistles to the Romans and the Hebrews.
(2) they are distinguished for boldness and vigor of style.
(3) they are written under great energy of feeling and of thought - a rapid and impetuous torrent that bears him forcibly along.
(4) they abound more than most other writings in parentheses, and the sentences are often involved and obscure.
(5) they often evince rapid transitions and departures from the regular current of thought. A thought strikes him suddenly, and he pauses to illustrate it, and dwells upon it long, before he returns to the main subject. The consequence is, that it is often difficult to follow him.
(6) they are powerful in reproof - abounding with strokes of great boldness of denunciation, and also with specimens of most withering sarcasm and most delicate irony.
(7) they abound in expressions of great tenderness and pathos. Nowhere can be found expressions of a heart more tender and affectionate than in the writings of Paul.
(8) they dwell much on great and profound doctrines, and on the application of the principles of Christianity to the various duties of life.
(9) they abound with references to the Saviour. He illustrates everything by his life, his example, his death, his resurrection. It is not wonderful that letters composed on such subjects and in such a manner by an inspired man produced a deep impression on the Christian world; nor that they should be regarded now as among the most important and valuable portions of the Bible. Take away Paul's letters, and what a chasm would be made in the New Testament! What a chasm in the religious opinions and in the consolations of the Christian world!
But his bodily presence - His personal appearance.
Is weak - Imbecile, feeble (ἀσθενὴς asthenēs) - a word often used to denote infirmity of body, sickness, disease; Matthew 25:39, Matthew 25:43-44; Luke 10:9; Acts 4:9; Acts 5:15-16; 1-Corinthians 11:30. Here it is to be observed that this is a mere charge which was brought against him, and it is not of necessity to be supposed that it was true, though the presumption is, that there was some foundation for it. It is supposed to refer to some bodily imperfections, and possibly to his diminutive stature. Chrysostom says that his stature was low, his body crooked, and his head bald. Lucian, in his Philopatris, says of him, "Corpore erat parvo, contracto, incurvo, tricubitali" - probably an exaggerated description, perhaps a caricature - to denote one very diminutive and having no advantages of personal appearance. According to Nicephorus, Paul "was a little man, crooked, and almost bent like a bow; with a pale countenance, long and wrinkled; a bald head; his eyes full of fire and benevolence; his beard long, thick, and interspersed with gray hairs, as was his head," etc. But there is no certain evidence of the truth of these representations. Nothing in the Bible would lead us to suppose that Paul was remarkably diminutive or deformed; and though there may be some foundation for the charge here alleged that his bodily presence was weak, yet we are to remember that this was the accusation of his enemies, and that it was doubtless greatly exaggerated. Nicephorus was a writer of the sixteenth century, and his statements are worthy of no regard. That Paul was eminently an eloquent man may be inferred from a great many considerations; some of which are:
(1) His recorded discourses in the Acts of the Apostles, and the effect produced by them. No one can read his defense before Agrippa or Felix and not be convinced that as an orator he deserves to be ranked among the most distinguished of ancient times. No one who reads the account in the Acts can believe that he had any remarkable impediment in his speech or that he was remarkably deformed.
(2) such was somehow his grace and power as an orator that he was taken by the inhabitants of Lycaonia as Mercury, the god of eloquence; Acts 16:12. Assuredly the evidence here is, that Paul was not deformed.
(3) it may be added, that Paul is mentioned by Longinus among the principal orators of antiquity. From these circumstances, there is no reason to believe that Paul was remarkably deficient in the qualifications requisite for an orator, or that he was in any way remarkably deformed.
And his speech contemptible - To be despised. Some suppose that he had an impediment in his speech. But conjecture here is vain and useless. We are to remember that this is a charge made by his adversaries, and that it was made by the fastidious Greeks, who professed to be great admirers of eloquence, but who in his time confided much more in the mere art of the rhetorician than in the power of thought, and in energetic appeals to the reason and conscience of people. Judged by their standard it may be that Paul had not the graces in voice or manner, or in the knowledge of the Greek language which they esteemed necessary in a finished orator; but judged by his power of thought, and his bold and manly defense of truth, and his energy of character and manner, and his power of impressing truth on mankind, he deserves, doubtless, to be ranked among the first orators of antiquity. No man has left the impress of his own mind on more other minds than Paul.

For his letters, say they, are weighty and powerful - He boasts of high powers, and that he can do great things. See on 2-Corinthians 10:1-2 (note).
But his bodily presence is weak - When you behold the man, you find him a feeble, contemptible mortal; and when ye hear him speak, his speech, ὁ λογος, probably, his doctrine, εξουθενημενος, is good for nothing; his person, matter, and manner, are altogether uninteresting, unimpressive, and too contemptible to be valued by the wise and the learned. This seems to be the spirit and design of this slander.
Many, both among the ancients and moderns, have endeavored to find out the ground there was for any part of this calumny; as to the moral conduct of the apostle, that was invulnerable; his motives, it is true, were suspected and denounced by this false apostle and his partisans; but they could never find any thing in his conduct which could support their insinuations.
What they could not attach to his character, they disingenuously attached to his person and his elocution.
If we can credit some ancient writers, such as Nicephorus, we shall find the apostle thus described:
Παυλος μικρος ην και συνεσταλμενος το του σωματος μεγεθος· και ὡσπερ αγκυλον αυτο κεκτημενος· σμικρον δε, και κεκυφος· την οφιν λευκος, και το προσωπον προφερης, ψιλος την κεφαλην, κ. τ. λ.
Nicephor., lib. ii., cap. 17.
"Paul was a little man, crooked, and almost bent like a bow; with a pale countenance, long and wrinkled; a bald head; his eyes full of fire and benevolence; his beard long, thick, and interspersed with grey hairs, as was his head, etc."
I quote from Calmet, not having Nicephorus at hand.
An old Greek writer, says the same author, whose works are found among those of Chrysostom, tom. vi. hom. 30, page 265, represents him thus: - Παυλος ὁ τριπηχυς ανθρωπος, και των ουρανων ἁπτομενος· "Paul was a man of about three cubits in height, (four feet six), and yet, nevertheless, touched the heavens." Others say that "he was a little man, had a bald head, and a large nose." See the above, and several other authorities in Calmet. Perhaps there is not one of these statements correct: as to Nicephorus, he is a writer of the fourteenth century, weak and credulous, and worthy of no regard. And the writer found in the works of Chrysostom, in making the apostle little more than a pigmy, has rendered his account incredible.
That St. Paul could be no such diminutive person we may fairly presume from the office he filled under the high priest, in the persecution of the Church of Christ; and that he had not an impediment in his speech, but was a graceful orator, we may learn from his whole history, and especially from the account we have, Acts 14:12, where the Lycaonians took him for Mercury, the god of eloquence, induced thereto by his powerful and persuasive elocution. In short, there does not appear to be any substantial evidence of the apostle's deformity, pigmy stature, bald head, pale and wrinkled face, large nose, stammering speech, etc., etc. These are probably all figments of an unbridled fancy, and foolish surmisings.

For [his] letters, say (g) they, [are] weighty and powerful; but [his] bodily presence [is] weak, and [his] speech contemptible.
(g) He notes out those who were the cause of these words.

For his letters, say they, are weighty and powerful,.... These words contain the reason why he did not choose to say any more of his authority as an apostle to punish offenders, that he might give no occasion for such a calumny, some among them, or the false apostles, had cast upon him; that his epistles, referring particularly to his former epistle, and that part of it which respected the incestuous person, and his delivery to Satan, were blustering and thundering; were laden with sharp reproofs and severe menaces; were heavy with charges, were filled with great swelling words, with boasts of power and authority, and with threatenings what he would do, when he came among them; whereas when present, as at his first coming to them, he was mild and gentle, even to a degree of meanness and baseness, as they suggested; and so they concluded he would be, should he come again; and therefore his letters were not to be regarded:
but his bodily presence is weak, and his speech contemptible: he made a mean figure, being of a low stature, and having an infirm body: the account the historian (m) gives of him is this, that
"he had a small and contracted body, somewhat crooked and bowed, a pale face, looked old, and had a little head; he had a sharp eye; his eyebrows hung downwards; his nose was beautifully bent, somewhat long; his beard thick and pretty long; and that, as the hair of his head, had a sprinkling of gray hairs:''
hence one in Lucian (n) scoffingly says of him,
"when the bald headed Galilean met me, with his hook nose, who went through the air to the third heaven:''
though the words of this text rather regard his mind and mien than the make of his body; and suggest that he was not a man of that greatness of soul, and largeness of mind, not possessed of those abilities and gifts, and of that freedom of speech, and flow of words, his letters promised; but instead of that, was a man of a mean spirit, very abject and servile, and to be despised; his conduct weak, and carrying no majesty and authority with his presence, his words without weight, his language vulgar, and style neglected; and, upon all accounts, a person worthy of no notice, and not at all to be either feared or regarded.
(m) Nicephorus, l. 2. c. 37. (n) In Philopatr.

letters--implying that there had been already more letters of Paul received by the Corinthians than the one we have, namely, First Corinthians; and that they contained strong reproofs.
say they--Greek, "says one," "such a one" (2-Corinthians 10:11) seems to point to some definite individual. Compare Galatians 5:10; a similar slanderer was in the Galatian Church.
weak-- (2-Corinthians 12:7; 1-Corinthians 2:3). There was nothing of majesty or authority in his manner; he bore himself tremblingly among them, whereas the false teachers spoke with authoritative bearing and language.

His bodily presence is weak - His stature, says St. Chrysostom, was low, his body crooked, and his head bald.