God has reserved to Himself in every generation a remnant of covenanted physical Israelite believers, even while the Nation of Israel itself is in unbelief and scattered. Scripture makes this clear that this reservation is by the election of God's Grace and not of outward works that we can see. Apostle Paul gives the example of Elijah who felt Israel had wholly forsaken God but God corrected Him and showed Him 7,000 men were were being used of God to preserve faith within the nation. Paul then explains this remnant in Physical Covenanted Israel will always exist regardless of what the nation as a whole does.
We use this to teach that just as Elect Israel is persevered by God and retains God's promises and is not forsaken, so too we the church made up of all nations can be assured that there has always been a believing Elect church in existence, no matter how corrupt or evil the outward "church" may appear at times. Just as God saves and keeps the Elected individual believer in Christ, so too will He keep and preserve his Covenanted Church and Nation throughout time and eternity.