Romans 11:4

This is clear, for Paul shows that not only Elijah remained as one devoted to God who did not worship idols, but that there were many who remained faithful to God, just as there were not a few Jews who believed in Christ.

And when Elijah made this lament before God, hear what the divine oracles answer him: "I have left for myself seven thousand men who have not bowed their knees to Baal." Baal is the name of an idol, to whose worship that people had fallen away, having abandoned the law of God. But let us see what the very number of seven thousand men indicates. The number seven pertains to rest. For "on the seventh day God rested."

Nor did Samuel, who had done so many surprising things, disregard David the beloved of God: yet they were both prophets, and the one was high priest, and the other was king. And when there were only seven thousand holy men in Israel who had not bowed the knee to Baal, Elijah alone among them, and his disciple Elisha, were workers of miracles. Yet neither did Elijah despise Obadiah the steward, who feared God, but wrought no signs; nor did Elisha despise his own disciple when he trembled at the enemies. Moreover, neither did the wise Daniel who was twice delivered from the mouths of the lions, nor the three children who were delivered from the furnace of fire, despise the rest of their fellow-Israelites: for they knew that they had not escaped these terrible miseries by their own might; but by the power of God did they both work miracles, and were delivered from miseries. Wherefore let none of you exalt himself against his brother, though he be a prophet, or though he be a worker of miracles: for if it happens that there be no longer an unbeliever, all the power of signs will thenceforwards be superfluous. For to be pious is from any one's good disposition; but to work wonders is from the power of Him that works them by us...

Elijah did not know about these 7, until God revealed their existence to him. So it is not unbelievable that you too are ignorant of how many Jews have believed in the Savior.

"He hath not cast off His people, whom He foreknew." As though he said, I have with me three, five, or ten thousand. What then? has the people come to be three, five, or ten thousand? that seed that compared with the stars of heaven for multitude, or the sand of the sea? Is this the way you deceive us and put a cheat upon us, by making the whole people thyself and the few that are with thee; and didst thou inflate us with idle hopes, and say that the promise has been fulfilled, when all are lost, and the salvation comes down to a few? This is all bombast and vanity! we cannot away with such sophistry as this. Now, that they may not say this, see how in the sequel he proceeds to the answer, not giving the objection indeed, but before it grounding the answer to it upon ancient history. What then is the answer? "Wot ye not," he says, "what the Scripture saith of Elias? how he maketh intercession to God against Israel, saying, Lord, they have killed Thy prophets, and digged down Thine altars; and I am left alone, and they seek my life. But what saith the answer of God unto him? I have reserved to Myself seven thousand men, who have not bowed the knee to the image of Baal. Even so then at this present time also, there is a remnant according to the election of grace." What he means is nearly this. "God hath not cast off His people." For had He done so, He would have admitted none of them. But if He did admit some, He hath not cast them off. Still it is said, if He had not cast off, He would have admitted all. This does not follow; since in Elijah's time the part to be saved had come down to "seven thousand:" and now also there are probably many that believe. But if you do not know who they are, this is no wonder, for that prophet, who was so great and good a man, did not know. But God ordered things for Himself when even the prophet knew them not. […]