Under the New Covenant God has put His law on our hearts, that is, through the Holy Spirit we have the gift of faith and the desire to do God's will.
Although we are justified by faith in Christ's atoning sacrifice and not by obedience to the law, nevertheless, we delight in God's law for it tells us what pleases God and helps us to identify which of our desires come from our sinful nature and which come from the Holy Spirit.
We will reply to those passages that the adversaries quote in order to prove that we are justified by love and works. They quote 1 Corinthians 13:2, "If I have all faith … but have not love, I am nothing." Paul testifies to the entire Church (they say) that faith alone does not justify. This passage of Paul requires love. We also require this. For we have said that renewal and beginning to fulfill the Law must exist in us, according to Jeremiah 31:33, "I will put My law within them, and I will write it on their hearts." If anyone should cast away love, even though he has great faith, he does not keep his faith, for he does not keep the Holy Spirit. In this passage Paul does not talk about the way of justification. Instead, he writes to those who, after they had been justified, should be urged to bring forth good fruit lest they lose the Holy Spirit. They quote this one passage, in which teaches about fruit [of faith]. Yet they leave out many other passages in which he discusses the way of justification. Besides, they add a correction to the passages that speak of faith, namely, that the passages ought to be understood as applying to "faith formed by love." They add no correction that there is also need for faith, which believes we are counted righteous for Christ's sake as the Atonement. The adversaries exclude Christ from justification and teach only a righteousness of the Law. (paragraphs 97-100)
Condensed from CONCORDIA: THE LUTHERAN CONFESSIONS, copyright 2005,2006 by Concordia Publishing House. Used by permission. All rights reserved. To purchase a copy of CONCORDIA, call 800-325-3040.