But if you call yourself a Jew and rely on the law and boast in God  and know his will and approve what is excellent, because you are instructed from the law; (ESV)
I remember when personal home computers first became a thing. I remember the Commodore 64 computers, and others that followed. During that time people learned how to write some basic computer commands for their home computers. I remember using a lot of "If - Then" statements. In this passage of Scripture Paul is writing a big "If - then" statement. He starts with "If you call yourself a Jew."
Over the next few verses we will see Paul building his "If" to be followed by a "Then" in verse 21. Let's look at the beginning part of his "If" statement. If you call yourself a Jew, and you rely on the law and boast in God, and know his will, and approve what is excellent. Paul is alluding to the arrogance of those who claim to follow the law.
Paul knew of what he was talking about. He refers to himself in his letter to the Galatians as a pharisee of pharisees. He was as close to perfect obedience to the Old Testament Law as anyone could be. What was the result? He was proud and arrogant. He was misguided to the point of missing who Jesus was and persecuting his followers.
If we are going to try to keep the Law we will always fail. Yet, because the Law demands absolute flawless obedience we are not going to want to be exposed for our failure. This leads to phony people who appear to be followers of the Law, but secretly are overwhelmed by failure. This is something that Paul will make clear in the "Then" statement that follows.
Pride goes before a fall. Be careful to put your hope in nothing less than Jesus' blood and righteousness. He alone is perfect and fulfilled the Law for you. "IF" you follow him, "THEN" you will never be disappointed.