Here "faith" is an abstract noun that refers to the person believing in God. The "person" here is any person. This can be translated in active form. Alternate translation: "God justifies any person who believes in God" or "when God justifies a person, he does so because the person believes in God" (See: [[rc://en/ta/man/translate/figs-abstractnouns]] and [[rc://en/ta/man/translate/figs-activepassive]])