Lewis’ religion was reduced to the only level Hollywood can muster: vague ramblings about “faith” and “God” and “another realm”. And since there was little actual Christianity, there was little intellectual content to Lewis’ character, and so the screenwriters had no raw material for the fierce friendly swordplay we associate with the Inklings.
Take away the Incarnation and Vicarious Suffering and Resurrection of the Body and there are no swords and nothing to fight or play about. So the screenwriters had no story, other than two nice people falling in love, her dying, and he trying to figure out how to go on. That’s poignant, but it is a common story. It was not Jack and Joy’s story, which was aparently too scandalous in its cruciform particulars to keep in the script.