It is the Resurrection that makes Christianity and stand out. It is why it is not actually a religion as people do not have to do anything they merely have to accept it it.
When Jesus rose from the dead on the third day (Sunday) Everyone knew exactly what had occurred. Thomas put is best when he stated My Lord and My God. Only a person without sin could rise from the dead. As EVERY man is sinful then it could only be god as man who rose.
How do we know this occurred. We there were plenty of witness, As Paul put it to people there were over 500 hundred, It would be very easy to seek them out to ensure the veracity of the statement.
The Jewish religious elite would have made hay with such an extravagant 'trumpian' claim if it were untrue but they didn't.
His Apostles who feared for their lives whilst hiding were then emboldened to speak out knowing death was certain and it was for most. Why die for a lie?
The significance of the resurrection is that the Crucifixion then made sense, Only a sinless person could take the punishment for others. Only a sinless man could be the perfect substitute.
This offer of being forgiven if a person repented of their sins was totally alien then and now to most people. It meant a person's action did not determine their entry into Heaven only their acceptance of Jesus as their saviour.
We should also note both Peter and Paul put the blame of Jesus's death fully on the Jews. Modern Day Jews do not deserve any opprobrium over this as it was God ordained. The other reason Modern day Jews are simply not related to the Jews of Biblical times. It is the Christians that are. That is why they follow Jesus.
The term christ killer is simply the statement of a biblically illiterate person