Paul has good reason for calling the minister of the Law the minister of sin, for the Law reveals our sinfulness. The realization of sin in turn frightens the heart and drives it to despair. Therefore all exponents of the Law and of works deserve to be called tyrants and oppressors.
The purpose of the Law is to reveal sin. That this is the purpose of the Law can be seen from the account of the giving of the Law as reported in the nineteenth and twentieth chapters of Exodus. Moses brought the people out of their tents to have God speak to them personally from a cloud. But the people trembled with fear, fled, and standing aloof they begged Moses: “Speak thou with us, and we will hear: but let not God speak with us, lest we die.” The proper office of the Law is to lead us out of our tents, in other words, out of the security of our self-trust, into the presence of God, that we may perceive His anger at our sinfulness.
All who say that faith alone in Christ does not justify a person, convert Christ into a minister of sin, a teacher of the Law, and a cruel tyrant who requires the impossible. All merit-seekers take Christ for a new lawgiver.
In conclusion, if the Law is the minister of sin, it is at the same time the minister of wrath and death. As the Law reveals sin it fills a person with the fear of death and condemnation. Eventually the conscience wakes up to the fact that God is angry. If God is angry with you, He will destroy and condemn you forever. Unable to stand the thought of the wrath and judgment of God, many a person commits suicide.
Martin Luther: Commentary on the Epistle to the Galatians (1535)
Trans. Theodore Graebner