John Calvin Commentary Romans 9:29

"And, as Isaiah hath said before, Except the Lord of Sabaoth had left us a seed, We had become as Sodom, and had been made like unto Gomorrah." — Romans 9:29 (ASV)

And as Isaiah had before said, etc. He brings another testimony from the first chapter of Isaiah, where the Prophet deplores the devastation of Israel in his time; and since this had happened once, it was not a new thing. The people of Israel indeed had no pre-eminence, except what they derived from their ancestors. These ancestors, however, had been treated in such a manner that the Prophet complained they had been so afflicted that they were not far from being destroyed, as Sodom and Gomorrah had been.

There was, however, this difference: a few were preserved for a seed, to raise up the name, so that they might not perish entirely and be consigned to eternal oblivion. For it was fitting for God to be ever mindful of His promise, so as to manifest His mercy in the midst of the severest judgments.