A common erroneous belief in Christianity is that the Old Testament is focused on the law and works, and the New Testament is when God introduces salvation by faith alone. Someone who believes this will often compartmentalize God, describing the Old Testament God as “just” and the New Testament God as “merciful.”
But the context of the Ten Commandments and God’s law is saturated in mercy. Before He gives the first command, God gives this preface: “I am the Lord your God, who brought you out of the land of Egypt, out of the house of slavery” (Exodus 20:2).
Remember, God brought His people out of bondage before giving them the law. His mercy and favor were not conditioned on their obedience.
God did not rescue His people because of their obedience, He commands their obedience because of what He has already done for them.
In the same way, our desire to obey God does not stem from an attempt to earn His favor, but in response to the mercies He lavishes upon us every day.
God prefaces His law with this reminder to demonstrate to His people that their hope does not come from the law, but from its Author, and we also need that reminder today. Legalism attempts to elevate the law over its Author and makes our hope of salvation reliant on our own abilities. But God essentially says here, “I have already rescued you; I have already delivered you. I have already called you and made you Mine, so obey me in these ways.”
In his book, Transforming Grace, Jerry Bridges explained the difference this way: “Under a sense of legalism, obedience is done with a view to meriting salvation or God’s blessing in our lives. Under grace, obedience is a loving response to salvation already provided in Christ.”