Galatians 6:13

For even those who are circumcised do not themselves keep the law, but they desire to have you circumcised that they may boast in your flesh.

(v13, ESV)

These people who teach circumcision are teaching salvation by works. They say that you need to follow the law in order to be saved. Part of following the law is to be circumcised, so you need to be circumcised in order to be saved.

I testify again to every man who accepts circumcision that he is obligated to keep the whole law.

(Galatians 5:3, ESV)

These followers of the law are obligated to keep the whole law. It's not just circumcision, but they also need to follow every other law. If they break even one single commandment in the law, they break the entire law. They don't even keep the law even though they are circumcised. They break the law in one way or another and do not follow the law entirely. As a result, they are hypocrites. They tell others to follow the law, but they themselves do not follow it. How can you believe and follow their teaching if their teaching is based upon hypocrisy. It doesn't make sense.

In the last verse, Paul explains why they do this. They do this in order to avoid persecution. Another result of their teaching of circumcision is so that they may boast in the flesh of others. They live by the flesh and boast in their own flesh. However, that's not enough. They also want to boast in the flesh of others. Other people who subscribe to their false teaching are ones who they can boast in. They do this so that they can be proud of those who follow their false teaching. Do not be deceived.