Here's a comment from John Calvin in regard to Galatians 2:11-21, (Paul's rebuke of Peter):
At the same time, this is a reply to another calumny, that Paul was but an ordinary disciple, far below the rank of an apostle: for the reproof which he administered was art evidence that the parties were on an equal footing. The highest, I acknowledge, are sometimes properly reproved by the lowest, for this liberty on the part of inferiors towards their superiors is permitted by God; and so it does not follow, that he who reproves another must be his equal. But the nature of the reproof deserves notice. Paul did not simply reprove Peter, as a Christian might reprove a Christian, but he did it officially, as the phrase is; that is, in the exercise of the apostolic character which he sustained.I was curious as to locating any sources (particularly contemporary sources of Calvin) that argued "Paul was but an ordinary disciple, far below the rank of an apostle." I did a short cursory search of a few sources in my personal library and Google, and didn't find anything. It would be interesting to know which source Calvin had in mind.
This is another thunderbolt which strikes the Papacy of Rome. It exposes the impudent pretensions of the Roman Antichrist, who boasts that he is not bound to assign a reason, and sets at defiance the judgment of the whole Church. Without rashness, without undue boldness, but in the exercise of the power granted him by God, this single individual chastises Peter, in the presence of the whole Church; and Peter submissively bows to the chastisement. Nay, the whole debate on those two points was nothing less than a manifest overthrow of that tyrannical primacy, which the Romanists foolishly enough allege to be founded on divine right. If they wish to have God appearing on their side, a new Bible must be manufactured; if they do not wish to have him for an open enemy, those two chapters of the Holy Scriptures must be expunged.
I have a copy of John Eck's Enchiridion of Comonplaces, and here's how he argues:
"Paul reproved Peter, because he was holding onto the edification of faith, that is, to the office of the apostolate in which they were equals, yet Peter was still prior in rule and authority. For even today it happens that the Pope and other superiors are often reproved by inferiors (Enchiridion of Comonplaces, p. 39).A most fascinating Roman Catholic commentary on this Galatians text comes from Haydock's Catholic Bible Commentary, 1859 edition. Now, if ever a text needed an infallible interpretation to settle the matter for Roman Catholics, this is it (note the interpretations of Jerome and Chrysostom!).
Ver. 11. But when Cephas, &;c. In most Greek copies, we read Petrus, both here and ver. 13. Nor are there any sufficient, nor even probable grounds to judge, that Cephas here mentioned was different from Peter, the prince of the apostles, as one or two later authors would make us believe. Among those who fancied Cephas different from Peter, not one can be named in the first ages[centuries], except Clemens of Alexandria, whose works were rejected as apocryphal by Pope Gelasius. The next author is Dorotheus of Tyre, in his Catalogue of the seventy-two disciples, in the fourth or fifth age[century], and after him the like, or same catalogue, in the seventh age[century], in the Chronicle, called of Alexandria, neither of which are of any authority with the learned, so many evident faults and falsehoods being found in both. St. Jerome indeed on this place says, there were some (though he does not think fit to name them) who were of that opinion; but at the same time St. Jerome ridicules and rejects it as groundless. Now as to authors that make Cephas the same with St. Peter, the prince of the apostles, we have what may be called the unexceptionable and unanimous consent of the ancient fathers and doctors of the Catholic Church, as of Tertullian, who calls this management of St. Peter, a fault of conversation, not of preaching or doctrine. Of St. Cyprian, of Origen, of the great doctors, St. Jerome, St. Augustine, St. Chrysostom, St. Gregory the Great, of St. Cyril of Alexandria, of Theodoret, Pope Gelasius, Pelagius the second, St. Anselm, St. Thomas Aquinas. In later ages, of Bellarmine, Baronius, Binius, Spondan, of Salmeron, Estius, Gagneius, Tirinus, Menochius, Alex Natalis, and a great many more: so that Cornelius a Lapide on this place says, that the Church neither knows, nor celebrates any other Cephas but St. Peter. Tertullian and most interpreters take notice, that St. Peter's fault was only a lesser or venial sin in his conduct and conversation. Did not St. Paul on several occasions do the like, as what is here laid to St. Peter's charge? that is, practise the Jewish ceremonies: did not he circumcise Timothy after this, an. 52[A.D. 52]? did he not shave his head in Cenchrea, an. 54? did he not by the advice of St. James (an. 58.) purify himself with the Jews in the temple, not to offend them? St. Jerome, and also St. Chrysostom, give another exposition of this passage. They looked upon all this to have been done by a contrivance and a collusion betwixt these two apostles, who had agreed beforehand that St. Peter should let himself be reprehended by St. Paul, (for this they take to be signified by the Greek text) and not that St. Peter was reprehensible; so that the Jews seeing St. Peter publicly blamed, and not justifying himself, might for the future eat with the Gentiles. But St. Augustine vigorously opposed this exposition of St. Jerome, as less consistent with a Christian and apostolical sincerity, and with the text in this chapter, where it is called a dissimulation, and that Cephas or Peter walked not uprightly to the truth of the gospel. After a long dispute betwixt these two doctors, St. Jerome seems to have retracted his opinion, and the opinion of St. Augustine is commonly followed, that St. Peter was guilty of a venial fault of imprudence. In the mean time, no Catholic denies but that the head of the Church may be guilty even of great sins. What we have to admire, is the humility of St. Peter on this occasion, as St. Cyprian observes, who took the reprehension so mildly, without alleging the primacy, which our Lord had given him. Baronius held that St. Peter did not sin at all, which may be true, if we look upon his intention only, which was to give no offence to the Jewish converts; but if we examine the fact, he can scarce be excused from a venial indiscretion. (Witham) --- I withstood, &c. The fault that is here noted in the conduct of St. Peter, was only a certain imprudence, in withdrawing himself from the table of the Gentiles, for fear of giving offence to the Jewish converts: but this in such circumstances, when his so doing might be of ill consequence to the Gentiles, who might be induced thereby to think themselves obliged to conform to the Jewish way of living, to the prejudice of their Christian liberty. Neither was St. Paul's reprehending him any argument against his supremacy; for is such cases an inferior may, and sometimes ought, with respect, to admonish his superior. (Challoner)
 Ver. 11. That Peter and Cephas were the same, see Tertullian, lib. de præscrip. chap. 23, p. 210. Ed. Rig.; Origen in Joan. Ed. Græcè et Latinè, p. 381.; St. Cyprian, Epist. 71. ad Quintum, p. 120.; St. Jerome on this Ep. to the Galatians, as also St. Chrysostom; St. Augustine. See his epistles on this passage to St. Jerome.; St. Gregory, lib. 2. in Ezech. tom. 1, p. 1368.; Gelasius apud Labb. T. 4. Conc. p. 1217.; Pelagius, the 2d apud Labb. t. 5. p. 622.; St. Cyril of Alexandria, hom. ix. cont. Julianum, t. 6, p. 325.; Theodoret in 2. ad Gal. iv. 3. p. 268.; St. Anselm in 2 ad Gal. p. 236.; St. Thomas Aquinas, lib. 2. q. 103. a. 4. ad 2dum. --- St. Jerome's words: Sunt qui Cepham non putent Apostolum Petrum, sed alium de 70 Discipulis....quibus primum respondendum, alterius nescio cujus Cephæ nescire nos nomen, nisi ejus, qui et in Evangelio, et in aliis Pauli Epistolis, et in hac quoque ipsa, modo Cephas, modo Petrus scribitur....deinde totum argumentum Epistolæ....huic intelligentiæ repugnare, &;c.
 Ver. 11. St. Chrysostom by a contrivance, eikonomon. p. 730, &;c.
 Ver. 11. Kategnosmenos may signfiy reprehensus, as well as reprehensibilis; and he says it is to be referred to others, and not to St. Paul: all upo ton allon.
 Ver. 11. St. Cyprian, Ep. ad Quintum, p. 120. Petrus....non arroganter assumpsit, ut diceret se primatum tenere, &;c.
Addendum: Tidbits from Luther
Therefore Jerome and Erasmus do Paul an injustice when they take the words “to his face” to mean “only according to the outward appearance”; they maintain that Paul did not oppose Peter sincerely, but that he did so with complaisant pretense, since others would have been offended if he had remained completely silent. But “to his face” means “in his presence”; for he opposed Peter openly, not in a corner but in the very presence of Peter and with the entire church standing by. When he says “to his face,” this is aimed especially against those poisonous spirits who slander those who are absent but do not dare open their mouths in the presence of these people. That is what the false apostles did; he touches them obliquely here, because they did not dare slander him in his presence; they did so only in his absence. “I did not,” he says, “speak evil of Peter this way; but I opposed him candidly and openly, not because of any pretense, ambition, or other human affection or mental disease, but because he himself was deserving of attack.”
Luther, M. (1999, c1963). Vol. 26: Luther's works, vol. 26 : Lectures on Galatians, 1535, Chapters 1-4 (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (26:108). Saint Louis: Concordia Publishing House.