I don't think that christianity is nothing more than slave morality. But if you are into Nietzsche's will to power as a philosophical idea, then all 'religion', or spirituality, becomes...art, if you will. Art created by an organism with a specific form of life, which would take a different form if the organism was different, i.e. placed differently in the whole.
But, like you ask, what really is transcendence then. It is certainly not what Perennialists take it to be, metaphysically speaking. It still might be valuable, however.
(Note: this is not to reduce Nietzsche to a post-modernist philosopher. Nietzsche was, in my opnion, definetely a quasi-essentialist thinker. He wasn't an existensialist, believing that 'exstence preceeds essence' (i.e. a non-essentialist). Something preceeds existence for N: something grounds man and man bumps up and measures himself against something. However, the 'essence' for N is the will to power. Something we cannot know in advance, and so unlike a clearly existing anthropomorphic 'God'. The will to power is a field of force of shifting relations, based on turmoil, conflict, violence and becoming. (perhaps not unlike the view modern physicists hold? Another quesiton altogether).