Origen on Romans 11:6

"And he has added, 'But if grace, it is no longer on the basis of works.' [Romans 11:6] One should know that the works that Paul repudiates and frequently criticizes are not the works of righteousness that are commanded in the law, but those in which those who keep the law according to the flesh boast; i.e., the circumcision of the flesh, the sacrificial rituals, the observance of Sabbaths or new moon festivals. [Cf. Colossians 2:18] These, then, and works of this nature are the ones on the basis of which he says no one can be saved, [cf. Ephesians 2:8-9] and concerning which he says in the present passage, 'not on the basis of works; otherwise grace would no longer be grace.' [Romans 11:6] For if anyone is justified through these, he is not justified gratis. But these works are by no means sought from the one who is justified through grace; but this one should take care that the grace he has received should not be in him 'in vain,' as Paul also says, 'For his grace in me was not in vain but I worked harder than any of them.' And again he adds, as one mindful of grace, 'Not I, but the grace of God with me.' [1 Corinthians 15:10] So then, one does not make grace become in vain who joins works to it that are worthy and who does not show himself ungrateful for the grace of God. For anyone who sins after having attained grace becomes ungrateful to him who offered the grace."
(Origen, 'Commentary on Romans' 8:7:6, in 'Fathers of the Church' 104:159)

(My thanks to Errol Amey for bringing this quote to my attention)