St. Thomas Aquinas on Galatians 3:11-14

This is relevant not only to Scripture from the Office of Readings but also to the crisis in the Catholic Church. 

11 But, when Cephas was come to Antioch, I withstood him to the face, because he was to be blamed.

12 For, before that some came from James, he did eat with the Gentiles; but, when they were come, he withdrew and separated himself, fearing them who were of the circumcision.

13 And to his dissimulation the rest of the Jews consented; so that Barnabas also was led by them into that dissimulation.

14 But, when I saw that they walked not uprightly unto the truth of the gospel, I said to Cephas before them all: If thou, being a Jew, livest after the manner of the Gentiles and not as the Jews do, bow dost thou compel the Gentiles to live as do the Jews?

The Apostle showed above that he received nothing useful from the discussion held with the apostles; now he shows that he benefitted them:

First, he shows how he helped Peter by correcting him;

Secondly, he tells what he said (v. 12).

He says, therefore: Indeed, they advantaged me nothing; rather I conferred something upon them, and especially upon Peter, because when Cephas was come to Antioch, where there was a church of the Gentiles, I withstood him to the face, i.e., openly: “Reverence not thy neighbor in his fall and refrain not to speak in the time of salvation” (Sir 4:27). Or: to his face, i.e., not in secret as though detracting and fearing him, but publicly and as his equal: “Thou shalt not hate thy brother in thy heart: but reprove him openly, lest thou incur sin through him” (Lev. 19:17). This he did, because he was to be blamed.

But it might be objected: This took place after they received the grace of the Holy Spirit; but after the grace of the Holy Spirit the apostles did not sin in any way. I answer that after the grace of the Holy Spirit the apostles did not sin mortally, and this gift they had through the divine power that had strengthened them: “I have established the pillars thereof’ (Ps 74:4). Yet they sinned venially because of human frailty: “If we say that we have no sin,” i.e., venial, “we deceive ourselves” (1 John 1:8).

Apropos of what is said in a certain Gloss, namely, that I withstood him as an adversary, the answer is that the Apostle opposed Peter in the exercise of authority, not in his authority of ruling. Therefore from the foregoing we have an example: prelates, indeed, an example of humility, that they not disdain corrections from those who are lower and subject to them; subjects have an example of zeal and freedom, that they fear not to correct their prelates, particularly if their crime is public and verges upon danger to the multitude.