I don't see the covenant in phases, i.e. Israel as one, then everyone else second. It was spoken to the Jews first, yes but it was never limited to them. It was always for everybody, and there have always been believing non-Jews (Rahab is an example.) Plain reading of Jesus' words indicate that if we believe Israel is synonymous with a particular tribe of people related by blood and a nation, then we are mistaken ... we are thinking with the mind of the flesh, and not the mind of the spirit. We are looking at the outside appearance (1 Samuel) and not at the heart. There have been many people born into Israel who are not believers, and will not partake in the only way by which anyone can be saved. And there are many people not born into Israel who are still yet receiving the promise made to Israel (or Paul is a liar). So we're left with realizing that when Jesus talked about not all in Israel are Israel, he was separating the physical (genetic, tribal, blood) from the spiritual (believers). I'm left to conclude that from the beginning, there has only been one promise and while it was given to particular tribe to proclaim to the nations, that tribe has no special category of salvation, no dispensation, or outpouring of special grace, than me or you.
Replacement theology views baptism is the replacement for cirumcision ... if circumcision made you part of Israel, then baptism today makes you part of Israel (i.e. God's chosen people). Its ironically a carry forward from dispensational theology which believes the circumcised have one plan of salvation, and the rest of us have a different one - a parallel one. But Paul says Baptism is for the individual to identify with the death burial and resurrection of Jesus Christ. Neither Jew nor Greek matters then for those who have identified with him and are in Christ. There has only every been one plan of salvation, for anyone, revealed first to a particular group of folks who were supposed to in turn reveal it to everyone else, but did not. With the veil torn, all of us are entrusted with the message of reconciliation, but many of us do not.
The only blood line that matters is that of Adam's. The only covenant is the Gospel which was repeated to Adam, Noah, Abraham, Moses, David, etc. Hebrews tells us that those who believed the one Gospel in each repetition by faith are saved, regardless of tribe, nationality, or blood.
