Romans 11:6

But if it is by grace, it is no more of works: otherwise grace is no more grace.

No more of works ... means "not of fleshly descent," as the expression is used in Romans 9:11, which see, especially the quotation from John Murray. The great objection to Paul's preaching the gospel of Christ, on the part of the old Israel, had to do with his categorical rejection of all the elaborate ceremonial of Moses' law, to which the fleshly Israel tenaciously clung, not in the sense of keeping it, as did Zacharias and Elizabeth, but in the sense of making it a device of their own glorification; and, upon such a basis, they denied that salvation could be extended to Gentiles. Further, the glaring fact that Paul had just shown that the righteous remnant, both in Elijah's day and presently, had obeyed God, the former by not bowing to Baal, the latter by obeying the gospel, and the equally glaring fact and even notorious fact of the fleshly Israel's thinking that salvation could be "earned" through the devices they followed, coupled with Paul's passion to show that salvation was never, either then, nor previously, nor now, nor ever, something people could earn or merit - all this prompted Paul here to pause and stress again the great doctrine of grace. R. L. Whiteside has a perceptive paragraph on this as follows:

There is no grace when a man merits salvation. Works by which a man merits justification and commands which one must obey to be saved are distinct matters. It is unfortunate that many cannot, or will not, see this distinction. Because of this, they conclude that a sinner must do nothing in order to be saved; but a man has no real understanding of either works or grace if he thinks that a sinner's complying with the terms of salvation causes him to merit it. Many things are of grace, and are yet conditional. Is anyone so simple as to think that Naaman's healing from leprosy was any less a matter of grace because he had to dip seven times in the Jordan river? Is any so blind that he cannot see that Jesus' giving sight to the man born blind was any less of grace because he was required to wash in the pool of Siloam?[8]