Adam Clarke on Romans 4:2

For if Abraham were justified by works - The Jew proceeds: - I conclude, therefore, that Abraham was justified by works, or by his obedience to this law of circumcision; and, consequently, he has cause for glorying, καυχημα, to exult in something which he has done to entitle him to these blessings. Now, it is evident that he has this glorying, and consequently that he was justified by works.

Apostle. But not before God - These seem to be the apostle's words, and contain the beginning of his answer to the arguments of the Jew, as if he had said: - Allowing that Abraham might glory in being called from heathenish darkness into such marvellous light, and exult in the privileges which God had granted to him; yet this glorying was not before God as a reason why those privileges should be granted; the glorying itself being a consequence of these very privileges.