“So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit.” –Matthew 7:17-18
You say: Since forgiveness depends on faith alone, why must one nonetheless do good works? Answer: If faith is of the true sort, it cannot be without good works, just as no good work can be where unbelief dwells. Therefore, if you believe, it is inevitable that good works will follow from faith. For just as faith brings brings salvation and eternal life, so it also brings good works and cannot be stopped from doing so. A living person cannot help busying himself, eating, drinking, and having something to do; nor can such works be lacking as long as a man lives. Therefore, it is not necessary to command and drive a man to preform such acts; as he will do them naturally, as long as he is alive. In like manner, no more is required to have one do good works than to say: Only believe; then you will do everything of your own accord.
Therefore, you need not demand good works from the man who believes, for faith teaches him all this; then everything he does is done well, and his actions are nothing but precious good works, no matter how insignificant they are. For faith is so precious that it makes everything good.