In Provers 3:5-7, we have a choice between leaning on our own understanding of right and wrong by doing what is right in our own eyes or trusting in God with all of our heart by doing what He has instructed, and He will make our way straight, and this is what it means to have faith alone. God is trustworthy, therefore what He has instructed is also trustworthy (Psalms 19:7), so the way to trust in God is by obediently trusting in what He has instructed while it would be contradictory for someone to think that we should trust in God with all of our heart, but not trust in what He has instructed. Luther said that an idle faith is not a justifying faith, so he would be against certain understandings of what it means to have faith alone.