Something here sparked my curiosity. Based on a little tidbit of information here, how can one character be "tricked" into worshipping the evil deity, exactly? And wouldn't said deity be able to recognize that the faith isn't genuine, especially when it came to allocating power to a priest? And if deities in Forgotten Realms can be simply tricked like that, and aren't morally superior to mortals in any capacity, then what's the point of even worshipping a deity in a such a fantasy world?
That's a cool question. I can't answer as to the details of divine trickery, but I would assume it has something to do either an evil deity posing as something else, perhaps faking benign intent and setting up a slippery slope of some kind, or else offering a cunningly worded magically binding contract, or a clever misrepresentation of what they're actually up to. There's probably a source book out there somewhere that tells the story of how Sharess fell under Shar's dominion. I'll have to look for that later, because I'm curious, too.
This post is because I have an opinion about the point of worship in Forgotten Realms. First, the obvious - the deity grants powerful spells to favored followers. That would be reason enough to many people.
Secondly, one would worship because one believes in the values of the deity's portfolio. If I believe in justice, truth, protecting the innocent, and that sort of thing, I would naturally be drawn to churches of Tyr and Torm, because I'd want to be part of the community that supports and defends those values. If what I care most about is working the land and farming, I'd naturally worship Chauntea. If my highest values were new beginnings, redemption, and light, Lathander represents those things. I could go on, but you probably get the idea. As the deity's power grows through the worship of its followers, so are the values of the portfolio supposed to be magnified.
Thirdly, in Forgotten Realms lore, everyone needs a deity to "sponsor" them for a pleasing afterlife, so they don't wind up in the Wall of the Faithless. I find that whole spiel very problematic, just like ideas about Heaven and Hell motivating people in real life religion, but in the Forgotten Relams setting, afterlife does matter very much to people.
Sometimes it baffles me though, why anyone would worship Shar, who desires the destruction of existence. I can only imagine that her followers are nihilists who believe that life as we know it is too painful, and not existing is better than existing, because the pain and suffering of life far outweigh any pleasure or joy from it. Maybe they're like goth or emo people who are drawn irresistibly to death, with the thoughts and images that go with it, perhaps without even knowing why, exactly they feel that way?
I can think of the Twilight's Hammer from WoW as a related example of evil cult thinking. They wanted to help Deathwing destroy Azeroth. To me, that's a variation on the theme of death cult worship in general, in another game.
Of course, we could take the tack that we just need evil cults and deities to set up conflict in stories, and they have no deeper meaning behind them than to give the heroes something to fight. But I do think that the more relatable and understandable the villains of a story, the better is the quality of the story.
EDIT: All I can turn up from a quick Google search is that Sharess was tempted for a time by "the darker side of pleasure", and fell to Shar that way. Apparently it brought her too close to Shar's portfolio, such that Shar was able to dominate her and try to subsume her divine essence. She was rescued by Sune, and now opposes Shar and other evil deities vehemently along with the good deities. Shar holds a grudge against her for this.
So, I guess part of the answer to "how do evil deities trick a person" would have to do with temptation. Pretty classic "deal with the devil" type stuff, and "if you go to bed with the devil, sooner or later you're going to have to..." well, you know.
