I don't believe will you find any writings prior to the 1800s that would state God works differently at different times with different people. God works exactly the same way in exactly the same manner with each person throughout time. He saved you and I exactly the same way He saved Abraham and Paul; through His grace by the faith He has given us. There has always been only ONE chosen people of God.
But there WERE Gentiles included from Adam to Christ. Naaman and Nebuchadnezzar are examples. There is no evidence they "kept the Law" but they certainly were believers. Nebuchadnezzar gave his personal testimony in scripture. They simply had a different set of problems but they were saved in the same fashion; through grace by faith.
What about the even bigger picture that I pointed out here?
When Abraham circumcised all the members of His house, gentiles were included in that covenant promise and identified with the Church.
When Moses was told to circumcise any stranger who wanted to participate in the Passover, gentiles were included in that covenant promise and identified with the Church (Exodus 12:). By the time Israel entered the promised land, its clear they are far from a genetically related group of people. Lots of gentiles were now part of the Church (Israel).
And the fact that the land prophecy of Ezekiel 47:21-23 included strangers is also evidence that the Church (Israel) was always intended to include gentiles.
God's people have always been identified primarily by covenant, not by race. That's the real basis for identifying Israel and the Church.
Dispensationalists miss this big picture.
4. This covenant of grace is frequently set forth in Scripture by the name of a testament, in reference to the death of Jesus Christ the Testator, and to the everlasting inheritance, with all things belonging to it, therein bequeathed.5. This covenant was differently administered in the time of the law, and in the time of the gospel: under the law, it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come; which were, for that time, sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation; and is called the old testament.
6. Under the gospel, when Christ, the substance, was exhibited, the ordinances in which this covenant is dispensed are the preaching of the Word, and the administration of the sacraments of baptism and the Lord's Supper: which, though fewer in number, and administered with more simplicity, and less outward glory, yet, in them, it is held forth in more fullness, evidence and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the new testament. There are not therefore two covenants of grace, differing in substance, but one and the same, under various dispensations.
Chapter 7