Galatians 5:2

Some men came down from Judea and began teaching the brothers, “Unless you are circumcised according to the custom of Moses, you cannot be saved.”

But some of the sect of the Pharisees who had believed stood up, saying, “It is necessary to circumcise them and to command them to keep the Law of Moses.”

Now I, Paul, myself plead with you by the gentleness and forbearance of Christ—I who am humble when face-to-face with you, but courageous toward you when absent!

And I testify again to every man who receives circumcision, that he is under obligation to keep the whole Law.

I, Paul, am writing this with my own hand, I will repay it (not to mention to you that you owe to me even your own self as well).

The one who believes in the Son of God has this witness in himself. The one who does not believe God has made Him a liar, because he has not believed in the witness which God has borne witness about His Son.