Many of the first Christians genuinely believed they witnessed Christ's physical resurrection from the dead (over 500 of them - 1 Cor 15:1-8). Including some who initially opposed it like Paul. The writings of the New Testament took place in the same generation as when it took place (early). And many early followers of Jesus were willing to die, suffering torture and execution, for what they saw. This is not the kind of reaction we would expect from them if this historic event were a late-developing legend, nor if it were a fraud. It was widespread, early, unexpected and sincere.
If you have not done so, I would encourage you to read the gospels for yourselves with this in mind. Remember, they were not just willing to die for some philosophical or moral principle, but for the testimony of Jesus' resurrection.