The moral law expressed in Exodus 20 and Deuteronomy 5 is the abiding moral law, the natural law, that was first revealed in creation and that is written on the consciences of all humans (see Romans 2:14–15). This is why the Apostle Paul appeals so freely to “nature” in Romans 1 and 2. He knows, as we should know, that all image bearers know in their conscience that God is, that he alone is to be worshiped, and that we owe certain duties to our neighbor.

For my entire Christian life, without exception, whenever the minister has read the Ten Commandments (the Decalogue) in the worship service—which I heard only when I began worshiping in the Reformed churches—he always begin with the words “I am the Lord your God…”. That, however, is not where the Ten Commandments begin.
The Ten Commandments actually begin in Exodus 20:3, not in verse 2. As a consequence of neglecting this truth, most Christians (especially in Reformed congregations) hear and read the Decalogue as including Exodus 20:2 (and Deut 5:6) but there is another way of reading the Decalogue. You may not be familiar with it. I did not encounter it for a number of years but our unfamiliarity with an old truth does not make that old truth wrong or alien to the Reformed tradition (e.g., Lutheran). The old way is to distinguish between the prologue to the Decalogue and the Ten Commandments themselves. The distinction is intrinsic to the text of the Decalogue itself. Consider Exodus 20:2–3:

“I am Yahweh your God, who brought you out of the land of Egypt, out of the house of slavery. “You shall have no other gods before me.

Verse 2 is a declaration of two gospel truths:

1.  Yahweh is their God and
2.  he has delivered them graciously and sovereignly from bondage.

Yahweh kept the promise he made to Abraham “to be God to you and to your children after you” (Gen 17:7). In Exodus 6:7 he repeated the essence of that promise: “I will take you to be my people, and I will be your God, and you shall know that I am Yahweh your God, who has brought you out from under the burdens of the Egyptians.” In Exodus 20:2 he announces that he is who he said he is and did what he promised to do. The God of Abraham, Isaac, and Jacob makes and keeps gracious promises and he has kept covenant of  grace with his church. He has delivered his church from bondage.

Verse 3 is not a declaration of good news. It is a commandment. In traditional Protestant terms, the law comes at Sinai not in its pedagogical use but in its normative use, i.e., the third use of the law (tertius usus legis). The effect is to say, “In light of all that I have graciously, freely done for you, here is what I expect as a consequence.” We were not saved because we met a condition. We were saved by grace alone, through faith alone (and even that faith is a gift; Eph 2:8–10). The obligations we gratefully take up under the third use of the law are consequent obligations. We seek to love God with all our faculties and our neighbor as ourselves because God first loved us.

Remember also, the Decalogue is not the ceremonial (religious) law nor is it the judicial law. It is the moral law. The religious and judicial laws could and would be abrogated in time, after their fulfillment at Golgatha. The moral law, however, is grounded in the immutable nature of God. Christ fulfilled the moral law for us, in our place, but he did not abrogate it as the norm of the Christian life. Our Lord Jesus re-stated the moral law in Matthew 22:37–40. The Apostle Paul re-stated the moral law in Romans 13 and in Ephesians chapters 4–6. This distinction between the judicial, religious, and moral law is known as the threefold division of the law. See the resources below on this.

The moral law expressed in Exodus 20 and Deuteronomy 5 is the abiding moral law, the natural law, that was first revealed in creation and that is written on the consciences of all humans (see Romans 2:14–15). This is why the Apostle Paul appeals so freely to “nature” in Romans 1 and 2. He knows, as we should know, that all image bearers know in their conscience that God is, that he alone is to be worshiped, and that we owe certain duties to our neighbor.

As I indicated above. This distinction between the gospel of Exodus 20:2 and the law that begins in verse 3 is not new. It is as old as the Reformation. Calvin treated Exodus 20:2 and parallel passages, e.g., Lev 19:36 this way:

I am the Lord your God. In these first four passages he treats of the same points which we have observed in the preface to the Law; for he reasons partly from God’s authority, that the law should be reverently obeyed, because the Creator of heaven and earth justly claims supreme dominion; and, partly, he sets before them the blessing of redemption, that they may willingly submit themselves to His law, from whom they have obtained their safety. For, whenever God calls Himself Jehovah, it should suggest His majesty, before which all ought to be humbled; whilst redemption should of itself produce voluntary submission (John Calvin, Commentaries on the Four Last Books of Moses Arranged in the Form of a Harmony, trans. William Bingham (Bellingham, WA: Logos Bible Software, 2010), 1.343).

Calvin followed Luther’s fundamental distinction between law and gospel as two categories or two kinds of divine speech. See the resources below for more on Calvin’s relationship to Luther and for more on the Reformed appropriation of Luther’s distinction between law and gospel. TLDR: Anyone who tells you that Calvin did not follow Luther on this understands neither Luther nor Calvin.

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