For what if some did not believe? Shall their unbelief make the faith of God without effect? God forbid: yea, let God be true, and every man a liar; as it is written, That Thou mightest be justified in Thy sayings, and mightest overcome when Thou art judged.- Romans 3:3, 4

The seed of Israel had great privileges even before the coming of Christ. God had promised by covenant that they should have those privileges; and they did enjoy them. They had a revelation and a light divine, while all the world beside sat in heathen darkness. Yet so many Jews did not believe; that, as a whole, the nation missed the promised blessing. A great multitude of them only saw the outward symbols, and never understood their spiritual meaning. They lived and died without the blessing promised to their fathers. Did this make the covenant of God to be void? Did this make the faithfulness of God to be a matter of question? “No, no,” says Paul, “if some did not believe, and so did not gain the blessing, this was their own fault; but the covenant of God stood fast and did not change because men were untrue.”

When God devised the great plan of salvation by grace; when He gave His own Son to die as the Substitute for guilty men; when He proclaimed that whosoever believed in Jesus Christ should have everlasting life; you would have thought that everybody would have been glad to hear such good news, and that they would all have hastened to believe it. Christ is so suitable to the sinner. Why does not the sinner accept Him? The way of salvation is so simple, so suitable to guilty men, it is altogether so glorious, so grand, that if we did not know the depravity of the human heart, we should expect that every sinner would at once believe the gospel and receive its boons. But, alas, some have not believed! When the great drama of human history shall have been played out, the net result will be that the ways of God shall be vindicated notwithstanding all the unbelief of men. ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/2255.cfm