It's definitely Christian, but much deeper and better than mere allegory. "Narnia" is allegory.
As well as Lewis’ Space Trilogy
Out of the Silent Planet
Perelandra
That Hideous Strength
I actually prefer The Space Trilogy (so called) by Lewis over Narnia (which I appreciate). It is adult fantasy.
That Hideous Strength is one of the most vivid and chilling portrayals of Good versus Evil I have ever read.
Still, LotR is my all-time favorite, no contest.