 Four possible explanations for this phrase. Some say it means from the Old Testament faith to the New Testament. Some indicate it's from the beginning of the faith to the end of our faith, the salvation of our souls, 1 Peter 1 and 9. Others believe it's from the initial faith to faith in the gospel, which apparently is a context. Others say it's the faithfulness of God and the faith we have that God is faithful. Whatever the view is, I believe it's the initial faith to the faith of the gospel.