Does Romans 3:3 mean "Faith of God" or "Faithfulness of God"? [God, Law, and Liberty]

This is a special ediction because I'm asking you to consider telling me if I am missing something about the meaning of the words "faith of God" in my understanding of Romans 3:3, "For what if some did not believe? shall their unbelief make the FAITH OF GOD without effect?" (KJV). I ask in sincerity because I noticed this morning that ALL translations since the KJV, even the NKJV, translate "faith of God" as "the faithfulness of God." As I thought about the difference, I began to think the "faithfulness-of-God"-translations may be missing something important that is in the KJV.