I do agree that the Call is the standing feature to the paladin. Something in them burns brighter then the sun and causes them - not forces them, but makes them desire to - serve the Good. Now, for each paladin, the reason for answering the Call might be different, and paladins themselves are not supernaturally good, which leads to character flaws potentially leading to them falling.
That said, I would go so far as to claim that the Call doesn't even need a god. People who've read Dresden Files can note that there's a buddhist and even an agnostic Knight of the Cross.
Speaking of which, I would argue that Michael is a perfect example of the NT archtype paladin.
My reading of Paladin legend and lore differs somewhat.
There was very little option in the "Paladins" of various eras- and some who heard The Call thought they were mad before they realized the incessant demands were of divine origin. Some were even punished. One does not lightly ignore the express will of the divine.
As for The Call not requiring a god...I can buy that in 3Ed+ D&D and even a few modern novels, but they are far and away the exception. In a faith that has a concept of the divine that does not recognize or include true deities, the Paladin is probably someone who has been made aware of a particular need that he or she knows- to a dead certainty- that it is his cosmic duty to satisfy.
Even the agnostic fits to a certain point, since they, by definition, are seeking but do not know the answer as to whether there is a divine force or not. To them, The Call may be a personal test of whether the divine exists at all. To use another phrase from christian theology- they may well be "acts, not faith" oriented. Their Code will be somewhat different, since their motivation is so highly internalized. And the consequences of their failures result in the loss of their power because they have lost faith in themselves.
Atheists, though...