Just declared
It is written: And whoever believes is justified by him from all things from which you could not be justified by the law of Moses (Acts 13:39).
The key to understanding this verse is the word "justified".
Justification is a legal term that means "to declare righteous" - it's not a question of actually making us righteous; it's a question of God declaring us righteous. And he does this for everyone who believes in Jesus Christ.
This verse tells us that we cannot be justified by the Law of Moses. In other words, we cannot be declared righteous by keeping the law. We are justified by faith in Jesus Christ alone. This is good news! It means we don't have to try to win God's favor by our good works. We're already justified by faith, and we can rest on that certainty. Praise God!