Philippians 3:9

We now heard what the apostle renounced; let us now see what he laid hold on, and resolved to cleave to, namely, Christ and heaven. He had his heart on these two great peculiarities of the Christian religion. I. The apostle had his heart upon Christ as his righteousness. This is illustrated in several instances. 1. He desired to win Christ; and an unspeakable gainer he would reckon himself if he had but an interest in Christ and his righteousness, and if Christ became his Lord and his Saviour: That I may win him; as the runner wins the prize, as the sailor makes the port he is bound for. The expression intimates that we have need to strive for him and after him, and that all is little enough to win him. 2. That he might be found in him (Phi 3:9), as the manslayer was found in the city of refuge, where he was safe from the avenger of blood, Num 35:25. Or it alludes to a judicial appearance; so we are to be found of our Judge in peace, Pe2 3:14. We are undone without a righteousness wherein to appear before God, for we are guilty. There is a righteousness provided for us in Jesus Christ, and it is a complete and perfect righteousness. None can have interest or benefit by it but those who come off from confidence in themselves, and are brought heartily to believe in him.

— Matthew Henry's Concise Commentary (public domain)

be found in him--"be found" at His coming again, living spiritually "in Him" as the element of my life. Once lost, I have been "found," and I hope to be perfectly "found" by Him (Luk 15:8). own righteousness . . . of the law-- (Phi 3:6; Rom 10:3, Rom 10:5). "Of," that is, from. righteousness . . . of God by faith--Greek, "which is from God (resting) upon faith." Paul was transported from legal bondage into Christian freedom at once, and without any gradual transition. Hence, the bands of Pharisaism were loosed instantaneously; and opposition to Pharisaic Judaism took the place of opposition to the Gospel. Thus God's providence fitly prepared him for the work of overthrowing all idea of legal justification. "The righteousness of faith," in Paul's sense, is the righteousness or perfect holiness of Christ appropriated by faith, as the objective ground of confidence for the believer, and also as a new subjective principle of life. Hence it includes the essence of a new disposition, and may easily pass into the idea of sanctification, though the two ideas are originally distinct. It is not any arbitrary act of God, as if he treated as sinless a man persisting in sin, simply because he believes in Christ; but the objective on the part of God corresponds to the subjective on the part of man, namely, faith.

— Jamieson-Fausset-Brown Commentary (public domain)