Galatians 5:4

but Israel, pursuing a law of righteousness, did not attain that law.

Therefore, let us fear, lest, while a promise remains of entering His rest, any one of you may seem to have fallen short of it.

seeing to it that no one falls short of the grace of God; that no root of bitterness springing up causes trouble, and by it many be defiled;

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.