John Chrysostom

"Being filled with the fruits of righteousness which are through Jesus Christ unto the glory and praise of God;" i.e. holding, together with true doctrine, an upright life. And not merely upright, but "filled with the fruits of righteousness." For there is indeed a righteousness not according to Christ, as, for example, a moral life. …

Homily on Philippians 2 ↗

Theophylact of Ohrid

That is, so that in accordance with the correctness of teaching you would also lead a blameless life. By righteousness the apostle means every virtue, or specifically mercy. I do not say that you should practice virtue or mercy imperfectly, but that you should be filled with them. And since the Greeks also thought to attribute virtue and mercy to themselves, he added "by Jesus Christ," since their deeds are not according to Christ, but for the sake of vainglory and people-pleasing. …

Commentary on Philippians ↗

Thomas Aquinas

As to the effects of grace he says, filled with the fruits of righteousness. Good works are the fruit: "The return (fruit) you get is sanctification and its end, eternal life" (Rom. 6:22). Or, the fruits of righteousness, i.e., the reward of justice, namely, the crown: "Henceforth there is laid up for me the crown of righteousness" (2 Tim. 4:8); "For the fruit of good labors is renowned" (Wis. 3:15). This is obtained through Jesus Christ, because all that we do is good through Him. "Apart from me you can do nothing" (Jn. 15:5). …

Commentary on Philippians ↗