I believe this ties to how "Jews" are defined in terms of a covenant.
1) Jews are those with a bloodline from the patriarchs
2) with male Jews circumcised on the eighth day after birth. The significance of this is that it signifies a switch from a gentile covenant to a Jewish covenant such that it looks as if the Jewish covenant is a born-with covenant. All gentiles, on the other hand, are born with a covenant tracing back from Noah. Every single human will thus be judged by a covenant instead of the absolute Law (which applies to the angels.
To put it another way, gentiles are all born with a covenant (Noah's covenant), Jews are born with another covenant (i.e., Mosaic covenant for today's Jews). This covenant (i.e., Mosaic covenant) is treated as if it's born with under the requirement that Jews switch from Noah's covenant to Mosaic covenant by the practice of circumcision on the eighth day (though today's practice lasts several months after birth)
3) Jews need to observe the Mosaic covenant the best they can and thus remain loyal to God
4) They don't stand in the way of Christianity
Only when the above 4 points are fulfilled, a Jew is considered a child of Abraham from a covenantal perspective. Bloodline alone won't decide if one is a Jew.
That said. The Jews are a chosen people called to be a witness of God. It is thus possible that in terms of Law and covenant a witness can be saved besides what a covenant applies to general humans. An example is David, he's a murderer and an adulterer in terms of the Mosaic Covenant. However, God saved him by setting him aside as a witness (i.e., a prophet). Paul (when he's Saul) stands in the way of Christianity (though he fulfilled the first 3 points well). God, on the other hand, has chosen him to be a witness for him to be saved. If he's not chosen, he's just another Pharisee choosing to be a stumbling block to the spreading of the Gospel.
Something impossible to us, God can make it possible.