Philippians 3:9

‘And be found in him, not having a righteousness of my own, even that which is of the law, but that which is through faith in Christ (or ‘the faithfulness of Christ'), the righteousness which is from God by faith,'

For in Christ he has been provided with a righteousness that surpassed any righteousness that he himself had built up, a righteousness that was total and complete, the very righteousness of Jesus Christ (2 Corinthians 5:21; 1 Corinthians 1:30; Romans 3:24). It was a ‘gift of righteousness' based on Christ's obedience (Romans 5:17; Romans 5:19). He had now been robed with the robe of righteousness and covered with the garments of salvation (Isaiah 61:10). He no longer therefore sought to call on or point to his own righteousness, a righteousness precariously built up by striving to obey the Law, but trusted wholly in the righteousness that had resulted solely from believing in Jesus Christ, the righteousness provided by God through faith. And as a consequence the only thing that he desires is to be found ‘in Him'. Christ is all that he needs.

‘Through the faithfulness of Christ.' Strictly speaking in Pauline literature pistis (faith, faithfulness) followed by a genitive always indicates the person whose pistis it is (compare e.g. Romans 3:3; Romans 4:16). That being so we would have to translate here ‘through the faithfulness of Jesus Christ', indicating that it is because He was obedient (Philippians 2:8) that we can be covered with His righteousness (Romans 5:19). It was His faithfulness that made Him all sufficient as a satisfactory sacrifice (compare Hebrews 10:5). Thus we have here the idea that it was because of His faithfulness that we can have the righteousness which is from God by faith.