' Jesus died to redeem Israel, not to redeem all the sinners in the world'

I don't think there's any doubt that Christianity was in the first instance a Jewish sect.  However, whether the Gentiles were included in the redemption was a crux early on, and quickly decided. Paul's epistles (from c. 50 AD on) clearly develop the "redemption of the Gentiles" view. Even though the Synoptic Gospels (Matthew, Mark, Luke, from the 60s AD on) insist on lining up the legendary life of Jesus with a mythical Messiah profile (including back-references to the Prophets), you can read (the resurrected Jesus appears to the disciples and gives them proof of his physical existence):

Luke,  ch 24, King James version

44: And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
45: Then opened he their understanding, that they might understand the scriptures,
46: And said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day:
47: And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem. (my bold)

Another point to remember is that the entire New Testament was written in the lingua franca Greek, not in the vernacular Aramaic or religious Hebrew.

I'm not at all surprised that evidence for a three-day resurrection Messiah myth should surface, it seems obvious to me that there was one, and it was part of the frame of interpretation used by the early Christians. But the very early passage from Jewish sect to universalist religion is all the same fairly well documented.

In any case, beyond that, I agree with those who say it won't make much difference to believers today. Either they habitually ignore history (and will go on doing so), or, if they're of the open-minded learned kind, they already know what I've sketched out above.