Galatians 4:29

And Sarah saw the son of Hagar the Egyptian, whom she had borne to Abraham, laughing in jest.

But the son by the servant-woman had been born according to the flesh, while the son by the free woman through the promise.

But I, brothers, if I still preach circumcision, why am I still persecuted? Then the stumbling block of the cross would have been abolished.

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.