Atheist paladin sounds like it could work if morality in itself had some kind of magic properties. By sticking closely to a vow, you would slowly be able to tap into that energy, kinda like weave. However in D&D, gods are the interface between morality and you so you have to be a paladin for one of those. Kinda like a democratic election when you think about it, your can elect a candidate that share your ideas and he will then grant you powers through the legislation.
