How serious is it to abandon the gospel of salvation by faith apart from works, for the gospel of salvation by faith plus works?

Paul compared it to idolatry:

But then, indeed, when you did not know God, you served those which by nature are not gods (Gal 4:8).

Before the Galatians came to faith in Christ, they were pagan idolaters, serving those “which by nature are not gods.” They served these gods in the hopes of gaining salvation.

Then, when Paul arrived in Galatia, preaching the gospel, they came to faith and therefore knew God:

But now after you have known God, or rather are known by God… (Gal 4:9a).

But with the Galatians’ turn to legalism, their knowledge was in jeopardy. God still knew them, but did they still know Him? Paul seemed unsure. Why? Because returning to the law meant returning to a form of idolatry:

…how is it that you turn again to the weak and beggarly elements, to which you desire again to be in bondage? (Gal 4:9b).

Notice the parallel between “those which by nature are not gods” and “the weak and beggarly elements.” For Paul, it’s all the same. It’s all bondage to idolatry. To the Galatians, adopting the Jewish law looked like they were making spiritual progress.

But isn’t that an outrageous comparison?

How could Paul realistically equate following the Law with pagan idolatry? After all, God revealed the Law, and it explicitly forbade idolatry!

The key, of course, is that God revealed the Law for a purpose—to reveal your sin (Rom 7:7) and to lead people to Christ (Gal 3:24). If the Law is used for a different purpose (e.g., for salvation), or taken as a substitute for the finished work of the cross, it becomes an idol.

Trusting in the Law for your salvation, instead of believing in Christ, is like trusting in an idol to save you. It is an act of idolatry.