After reading Psalms, Hebrews and the life of Abraham I think the church existed prior to Christ and was based on the same grace and faith that saves all who believe. If we as Christians are imputed with righteousness just as Abraham then grace existed for Abraham and us. It matters not what time it is when someone believes in the messiah, we look back in time and see him while Abraham and others looked forward in time and saw him. God is not bound by time and His salvation is not either.
I agree that everyone from Adam forward who is saved, got that way through faith, and by the grace of God. But if you’re saying that means Abraham, David, and other Old Testament believers are part of the Church, I have to disagree. Although it’s composed of both those who had been Jews and those who had been Gentiles, the Church has become nothing less than a new race of human, different in origin and destiny from any other group mentioned in the Bible. It began at Pentecost, will end at the rapture, and none who came before or will come after can claim membership. Paul wrote that God’s purpose was to create one new man out of the two (Ephesians 2:14-16) and that in Christ we are neither Jew nor Gentile (Galatians 3:28).