John Chrysostom
How much power these words contain! He says the same thing that Paul stated more fully in the Epistle to the Romans: If Abraham was justified by works, he has something to boast about, but not before God (Rom. 4:2).
Do you see that this serves more for the instruction of the Jews than for the justification of the Gentiles?
If he had spoken of this without a special occasion, perhaps he would not have expressed such an observation. But once he had received the occasion, he spoke without fear.
And see how everywhere the actions of enemies turn to the benefit of the apostles. If they had not given the occasion, neither this nor what follows would have been spoken.
From this they learn that even if the Gentiles had not wished to convert, they still should not be despised.
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Since he was speaking not about the circumcised, but about the Gentiles, notice what he does. One point, being gradually confirmed, became more demonstrative.
The other belonged to one examining whether salvation was possible under the law. He shows that they themselves are in danger if the law could not do what faith did.
If faith is lacking, destruction inevitably threatens them. He did not say, You do not believe, which would have been too grievous, especially since the matter itself proved this.