If the player is deliberately underming the thematic loading of the Power (to get slightly Pemertonian)

then... that would be bad. The game tells you very clearly what kind of character you're playing via class, build, powers etc. If you want to reskin it you need clear GM agreement to your proposal and an eye for potential problems.
That said, I don't think 'atheist Paladin' is a problem. The commie pinkos at the BBC once made a WW2 episode of Dr Who I saw, where the sea-vampires ignored the Anglican priest's cross and ate him - but were repelled by the Soviet army captain's hammer and sickle badge, because he was effectively channeling his own Faith in the Revolution. I'm fine with that approach - a PC Paladin channeling his own belief in Virtue to Valiant Strike - it would still be necessary that he believe in Valour as an ideal, though.