The best summary of a paladin I read.

By the far the best gamable description of a paladin I seen was given by Elisabeth Moon in her Deed of Paksenarrion series.

Paraphrased From page 579 of the Trade Paperback the Dead of Paksenarrion.

Most think being a holy warrior means gaining vast arcane powers, that they would be nearly invincible against any foe. But truth is that while Paladin are skilled at fighting, that was the least of their abilities. A quest might involve no fighting at all, or a battle against beings no steel could pierce.

Above all paladins show that courage is possible. It is easy enough to find reasons to give in to evil. War is ugly as many know. But we do not argue that war is better than peace; paladin are not that stupid. It is not peace when cruelty reigns, when stronger men steal from farmers and craftmen., when the child can be enslaved, or the old thrown out to starve, and no one lifts a hand. That is not peace: that is conquest and evil.

Paladins do not start quarrels in peaceful lands, never display their skills to earn applause. But we are the sword of good defending the helpless and teaching by our example that one person can dare greater force to break evil's grasp on the innocent. Sometimes that can be done without fighting, without killing, and that is best.

But some evil needs direct attack, and paladins must be able to do it, and lead others in battle. Wonder why paladins are so likable? It is important, we come to a town, perhaps, where nothing has gone right for a dozen years. Perhaps there is a temple there and sometimes there is not. The people are frightened, and they have lost trust in each other, in themselves. We may lead them into danger, some will be killed or wounded. Why should they trust us?

Because we are likable, and other people will follow us willingly. And that's why we are more likely to choose a popular adept as a candidate rather than the best fighters.

To me this is best damn summary of a D&D style paladin I ever read and the basis for how I referee them.