“The Law” can mean the commandments, or it can also mean the first five books of the Bible, so when he says “you who desire to be under the law, do you not listen to the law?” Paul’s about to show that it doesn’t make sense to desire to be under the law when you listen to what the law – the first five books of the Bible – actually teach us. Specifically, he’s going to turn to Genesis to illustrate his point. Although the Judaisers would’ve been horrified at the accusation, Paul is saying their problem was they weren’t being faithful to the Bible.
It’s interesting that Paul is writing to people who apparently desire to be under the law. He’s said so much up to this point in the letter about the problems with being under the law – how it imprisons us, how it’s a slave master – it seems strange that anyone would want to be under the law. It might have made sense if the false teachers had misled them into thinking they needed to be under the law in order to be saved, but to desire to be under it is something different. They liked the idea of being judged based on their performance. That might seem such an obvious error for Christians to make, but I don’t think we should be too quick to blame them; listening to people preaching works of the law as a way for us to make ourselves righteous can be appealing in a way. I mean, we’d be turned off by someone who just preached “thou shalt nots” all the time, but if he can give us more positive-sounding practical things to do, the idea of being able to say “I did this, that, and the other righteous deeds today” can be attractive. If that’s our motivation for trying to obey God’s law, we’re doing it wrong. We need to watch ourselves; do I desire to be judged on my performance? If so, listen to what the law says.
It’s also a subtle problem with the Galatians’ desires. They desire to be under the law. The law is a good thing. They wanted to be faithful to the Scriptures. The trouble was they hadn’t noticed that the way the Judaisers were teaching the law actually contradicted what the Bible teaches. Again this is a warning to you and me: just because someone uses the Bible doesn’t mean he’s teaching it faithfully. We need to listen to what the Bible itself says, not just go along with what other people say it’s about.