Notice the opposite of grace and faith in these verses. But if you don't try to work for your righteous standing with God, but trust him who justifies the ungodly, God's righteousness will be credited to you as a gift. So grace is the purpose of God to give you the righteousness and the promise that you do not deserve. That is why grace is the guarantee of the promise. It overrides our demerit.
Faith is the restful experience of the work of grace in our lives.
If we think of that first act of justifying faith in Christ, we can say faith is to grace what seeing is to light and what hearing is to sound and what waking up is to the alarm clock. Faith corresponds to grace the way tasting sweetness corresponds to honey on the tongue. I know this is not a common way of talking about faith today. But it was three hundred years ago. There is a widespread loss of understanding today about the Biblical nature of grace and faith.