Priests

The picture is simple, a mortal sacrifices stuff for the god, the god receives divine points and becomes stronger and when the god is stronger it can do more things. It can then grand more powers or make more havoc. The mortal can also get divine points for his god by another means too. He can for example try to attain more followers or by doing quests for the god. The point here is to get the god more powerful and at the same time hoping to gain the god's favor. When the god actually notices that the actually mortal is doing something he might then answer to the mortal's prayers if they are not demanding too much of the god's powers and when the god sees the mortal a very valuable mortal indeed the god can grand special powers that are appropriate for the god's motives. A god who likes people dying can give the mortal a special power to unleash a powerful bolt to kill people as many times he wants to, except that every time the mortal uses the special ability it drains the god's power and so if the mortal uses the power too much or if the mortal uses the power for not appropriate reasons the god can get angry and even kill the mortal.

If the god haves a religion then these levels of power are pre-defined, but however, the god usually is the who dedices the mortal's advancement, or if the religion is just too large for the god's to handle, he may have given his arch bishop or similar powers to advance people to new ranks without even asking permission. But again if the god comes to know that his powers are abused, you guess it.

To distinguish the priest as a dedicated profession or character class the god may not see it appropriate for the priest to do mundane things like being a powerful fighter (if the god doesn't care about fighting) or researching magic and similar things, the god may ask the mortal to end those before he can attain any or more special powers and this is quite usual. So it is not likely to see a high priest partying with other mortals and killing dragons. The high priest usually comes out of his deity's temple to convert people to his religion or to do some quest. This dedicating restriction is also true for those who worship more than one god. The gods are very jealous of their territory and if they see that their followers are worshipping some another god, they can become angry for those without second thought. There is, of course, an exception if those gods are friends to each other and shares similar motives.