Galatians 5:7

And I went up because of a revelation, and I laid out to them the gospel which I preach among the Gentiles, but I did so in private to those who were of reputation, lest somehow I might be running, or had run, in vain.

I have confidence in you in the Lord that you will adopt no other view. But the one who is disturbing you will bear his judgment, whoever he is.

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.