The same thing that fuels these pages. It's a cocktail of 3 ingredients.Suppose the Resurrection was proven false, what would explain the Gospels?
1) The desire to possess enough knowledge to control, or protect, one's world.
2) The ability of the mind to invent artificial answers out of the need to fill that void at all costs.... and
3) The hubris required to defend any threat against one's (or a group's) internal development of of points 1 and 2.
The desire for knowledge is like the desire for love. It makes one hungry to a fault and can create bad bedfellows. Just the thought that you possess the answers to something is self soothing in this oftentimes scary and capricious world. History is ripe with these examples and Christianity is just another one on the heap.