Jesus WAS a fraud unless he was actually the Jewish messiah. Jews did NOT / do NOT accept him as such. Non-Jews attempt(ed) to tell the Jews that Jesus was the Jewish messiah. Islam (the fastest growing religion in the world – another spin-off from Judaism) also regards Jesus a NOT the messiah.
He 'became the messiah' by being 'deified' as Icon for the new gentile (non-Jewish) religion after he was dead. Paul/Saul supposedly 'met' the deceased in a 'vision' (or hallucination, or delusion, or fantasy, or whatever it was) and gospel writers (whoever they were) told stories (written decades later) of 'miracles' and long-dead bodies coming back to life. There has been NO verifiable evidence presented to substantiate the tales of supernatural feats or 'miracles' attributed to Jesus.
Although those tales evidently appealed to gentiles far from Judea, people living in the area with an opportunity to actually experience Jesus were less impressed (including his own hometown) – and do not regard the tales as true or as proof that Jesus was anything more than a wandering Jewish preacher (perhaps with delusions of grandeur). Wannabe messiahs were evidently not uncommon in the era and were regularly dismissed as frauds by Jews.
This seems to upset those who choose to worship Jesus thousands of years later based on unverified tales told by Paul/Saul, gospel writers, and other religion promoters. They often become angry and hostile when their supernatural tales are disbelieved and challenged.
However, as indicated in this thread, Christianity is declining in areas other than Africa, South America, and parts of Asia. It is not surprising that some who have great emotional investment in the worship of Jesus become defensive rather than acknowledging that their religion is declining as society advances.
Perhaps younger generations can come to grips with the fact that Christians comprise less than a third of the wold population (in 2020) – and the fact that Islam is the world's fastest growing religion.
A high percentage of Christians are in Africa; and in just one more generation forty percent of the world's Christians are predicted to be in Africa. Christianity appears to have run its course and is declining in North America, Europe, Australia, and elsewhere. Becoming upset, defensive, and hostile will not affect the outcome – but will say something about you (generic term).
Let's get back to the OP. "WHY is Christianity declining in the US and other advanced / technological / educated nations?"
ANY of the thousands of "gods" proposed, imagined, worshiped, loved, feared, and/or fought over by humans MAY exist -- awaiting verifiable evidence