@CycloneHe 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
anything 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
so much more 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
the occult, 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.