John Chrysostom

But he comes to the things which are matters of choice, wherein we have the "rather." "As touching the law, a Pharisee; as touching zeal, persecuting the Church." But this is not sufficient; for it is possible to be a Pharisee even, and yet not very zealous. But this also he adds; behold the "rather." "According to righteousness." It is possible, however, to be adventurous, or to act thus from ambition, and not out of zeal for the law, as the chief priests did. …

Homily on Philippians 10 ↗

Theophylact of Ohrid

Since some of the Pharisees were not particularly zealous for the law, the apostle says that he was so zealous that he persecuted the disciples of Christ. Since many were zealots out of love of power and for other motives, and not for the sake of the law, the apostle says that, being blameless according to the righteousness of the law, how could I have been a persecutor for the sake of anything human, and not out of divine zeal.

Commentary on Philippians ↗

Augustine of Hippo

Nor let us be disturbed by what he wrote to the Philippians: "Touching the righteousness which is in the law, one who is without blame." For he could be within in evil affections a transgressor of the law, and yet fulfil the open works of the law, either by the fear of men or of God Himself; but by terror of punishment, not by love and delight in righteousness. …

On Two Letters Of Pelagius 1.15 ↗

Thomas Aquinas

Then he shows the prestige he had in his manner of life: first of all, in regard to his sect when he says, as to the law a Pharisee. For there were three sects among the Jews, namely, the Pharisees, the Sadducees, and the Essenes. But the Pharisees were closer to the truth, because the Sadducees denied the resurrection and did not believe in angels or spirits, while the Pharisees believed both, as it is stated in Acts (23). For this reason the sect of the Pharisees was more commendable. …

Commentary on Philippians ↗