Why is this debate over faith in Jesus Christ important?... because it reveals the emphasis in Paul's thinking. In other words, if these texts say that one must believe in Jesus Christ for righteousness, such a theme is incredibly pervasive and forceful in Paul's letters. He reminds his readers again and again that they must put their faith in Jesus Christ to stand in the right before God, to be saved on the last day.Faith Alone, 131-132
Grammatically, there are other places where the genitive refers to Christ as the object.
In Philippians 3:8, Jesus Christ is described as the object of knowledge. In 1 Thessalonians, he is described as the object of hope. In both these cases, it is clear from the context that Paul is not talking about Christ's knowledge or Christ's hope. There is no grammatical reason why the same can't be true of pistis christou.https://www.thegospelcoalition.org/blogs/trevin-wax/faith-in-christ-or-faithfulness-of-christ/