Just to go back to Luther because I may have given the impression that he does not think that what we believe is important. He did, but he wanted to emphasise that faith is more than just what we believe and, for him, it is fundamentally about trust. Faith is about trusting a God who makes promises and whose promises may be relied upon. So, for him, faith is a relational notion. It's about trusting God, trusting his promises and so receiving what is promised.
But he wasn't saying that faith is only about our personal attitude of trusting God. It's also about trusting a specific way of seeing ourselves and the world i.e. our beliefs, which for him was the Christian faith.