Here is the Lutheran definition of the Law:
Therefore [we shall set forth our meaning:] we unanimously believe, teach, and confess that the Law is properly a divine doctrine, in which the righteous, immutable will of God is revealed, what is to be the quality of man in his nature, thoughts, words, and works, in order that he may be pleasing and acceptable to God; and it threatens its transgressors with God's wrath and temporal and eternal punishments. For as Luther writes against the law-stormers [Antinomians]: Everything that reproves sin is and belongs to the Law, whose peculiar office it is to reprove sin and to lead to the knowledge of sins, Rom. 3:20,7:7; and as unbelief is the root and well-spring of all reprehensible sins [all sins that must be censured and reproved], the Law reproves unbelief also.
And from the same the definition of the Gospel:
For everything that comforts, that offers the favor and grace of God to transgressors of the Law, is, and is properly called, the Gospel, a good and joyful message that God will not punish sins, but forgive them for Christ's sake.
Solid Declaration of the Formula of Concord: Law and Gospel
