[@Cyclone]He tests people on a case-by-case basis.

Imagine a great sorcerer named Ashmedai. He has an unending thirst for power, to unravel the secrets of the universe, to rule over all in all his majesty. He makes a great deal with Mammon. He will sacrifice anything -- everything! -- for immense, near-endless power.

So Mammon takes his desire for power. He takes his need to unravel the secrets of the universe, and gives him a sense of wonder for the material world. And he grants this Ashmedai the power he wanted. And a reformed Ashmedai is left wandering the world as a peaceful traveller, able to wreak great works but seeing no need for such things.

This is just an example, with a few domains stretched to get my point across. This also only works like that if Ashmedai truly would give [i]anything[/i] for power. In this case, it is his resolve for power that is being tested; does he wish for power itself -- or control (the power to use power)?

It's ideals that are tested. An upstanding magical researcher, ardent worshipper of Belru and Vulamera, a pillar of the scholastic community -- convinced that he would be able to work out [i]so much more[/i] if he just burned a few books, broke a few deals, and gained the power of a knowledgeable demon. On the other hand, a career warlock would be someone who merely exploited the other material means of acquiring magic -- the Adversary is not [i]the occult[/i], the occult is merely his domain. You could smash a dragon's skull and burn a special herb and cast a herniating curse on your mother-in-law without contacting Mammon at all! But you wouldn't summon demons.

That, or someone who managed to be tempted further and further astray with every summoning. This would be a great king, or powerful priest; someone who would make "worse" and "worse" decisions. This could be for the better -- a Reathite warden convinced that maybe this young child shouldn't be killed this early, or a Logosite's resolve in imperial order shaken.