3:27-31 God will have the great work of the justification and salvation of sinners carried on from first to last, so as to shut out boasting. Now, if we were saved by our own works, boasting would not be excluded. But the way of justification by faith for ever shuts out boasting. Yet believers are not left to be lawless; faith is a law, it is a working grace, wherever it is in truth. By faith, not in this matter an act of obedience, or a good work, but forming the relation between Christ and the sinner, which renders it proper that the believer should be pardoned and justified for the sake of the Saviour, and that the unbeliever who is not thus united or related to him, should remain under condemnation. The law is still of use to convince us of what is past, and to direct us for the future. Though we cannot be saved by it as a covenant, yet we own and submit to it, as a rule in the hand of the Mediator.
30. it is one God who shall justify—"has unchangeably fixed that He shall justify."
the circumcision by—"of"
faith, and the uncircumcision through faith—probably this is but a varied statement of the same truth for greater emphasis (see Ro 3:22); though Bengel thinks that the justification of the Jews, as the born heirs of the promise, may be here purposely said to be "of faith," while that of the Gentiles, previously "strangers to the covenants of promise," may be said to be "through faith," as thus admitted into a new family.