But most English Bibles, including the NRSV and NIV, translate pistis christou ("faith of Christ") in Rom 3:21-22 and Gal 2:16 as "faith in Christ," to support the doctrine of "justification by faith." This is very problematic because I don't think Paul means by believer's faith in Christ. If he had meant it, he would have used the preposition en instead of the genitive case, like "pistis en christo," which appears frequently in the Deutero-Pauline and Pastoral Letters: Col 1:4; 1 Tim 1:4; 3:13; 2 Tim 1:13; 3:15. In these later epistles, authors clearly mean believer's faith in Christ.
But in Paul's undisputed letters, he points to Christ's faith first and then believer's participation in his faith. In Rom 3:22, he says that God's righteousness comes through Christ's faith for all who have faith [in my book (see below), Rom 3:22 presents a snapshot of Paul's threefold gospel, which contains three elements of participation: God's righteousness, Christ's faith, and believer's faith]. Similarly, the righteous one shall live by faith (Rom 1:17; c.f., Hab 2:4). In Rom 3:26, Paul says that God justifies the one who has "faith of Jesus," which is Jesus's faith. Also, in Gal 2:20, he says he wants to live by Christ's faith (not "by faith in Christ") because Christ lives in him.
In 2011, I wrote A Theological Introduction to Paul's Letters to check in with Paul's thought on the threefold gospel, in which I pointed out the importance of God's righteousness (not our righteousness), Christ's faith (not merely his death), and Christian participation in Jesus's faith (not merely salvific knowledge).