To call it “allegory” goes too far. Clarke doesn’t closely follow the Christian end-times ideas (and I’m not sure he would be following an American version at all). The Overlords aren’t rebel angels at all - they are willing servants of the Overmind.
The idea of future generations of humanity being subsumed into first a single racial mind, then a larger cosmic intelligence doesn’t fit well either (even if some Christians - usually tending to mysticism - adopt something resembling the latter view, it’s hardly explicit, of even clearly implicit, in the Bible).