When the Greek genitive case of pistis Iesou Christou is used in Paul’s letters, we should be careful about whose faith Paul talks about. Technically, the genitive phrase means either “believer’s faith in Jesus Christ” or “the faith of Jesus Christ.” But whose faith does Paul talk about? Please watch this video if you are really interested in Paul's theology of Christ and faith.
Quote from this video:
In Gal 2:16, the issue is by whose faith one is justified. The Greek genitive phrase, "pisteos Iesou Christou" means, technically, either “faith in Jesus Christ” or “the faith of Jesus Christ.” “Faith in Jesus Christ” means a believer’s faith in Jesus, which we call the objective genitive case (Jesus is the object of faith). “Faith of Jesus Christ” means Jesus’s faith, which we call the subjective genitive (Jesus is the subject of faith).
“Faith of Jesus Christ” makes better sense for two reasons: (1) Throughout Galatians and Romans as well, Paul first emphasizes Christ’s work and his faith and then asks believers to participate in Jesus’s faithfulness. Otherwise, if Paul had meant believer’s faith in Jesus Christ, he could have used the prepositional phrase, something like "pisteos en Iesou Christo"; (2) In Paul’s theology, it makes better sense that justification happens through Jesus Christ’s faithfulness, which involves believer’s participation in Christ’s faithfulness.