I’ve been dipping into the Common English Bible a bit and I really like the way it has translated hupakoēn pisteōs in Romans 1:5 and 16:26. They render the phrase as “faithful obedience,” so that Paul’s apostleship “was to bring all Gentiles to faithful obedience for his name’s sake.”
Further, they’ve got “faithfulness” throughout the remainder of the greeting so that “the news about [the Roman Christians’] faithfulness is being spread throughout the whole world.”
Moreover, Paul writes that they “can be encouraged by the faithfulness we find in each other, both your faithfulness and mine” (v. 12).
In 3:22, CEB prefers the subjective genitive of pisteōs Iēsou Christou: “God’s righteousness comes through the faithfulness of Jesus Christ for all who have faith in him.”
Whoever worked on Galatians, however, went in the opposite direction in translating pisteōs Iēsou Christou. In 2:15-21, the objective genitive is preferred, and “faith” rather than “faithfulness” appears throughout chapter 3.