Mormonism is fairly simple.
It started with this guy Joseph Smith
Who came from a family that believed dark magic and sold his services as a treasure hunter and crystal gazer (which he was sued over because he wasn't that good at it)
Anyhoo he goes and prays about which church to join. And he has a vision
Where he saw God and Jesus (Or just God or just Jesus. It varies, Joseph told about 9 different versions of this throughout his life, which Mormons believe makes it much more believeable) and God told him all the other religions were wrong and he should start his own.
Fast forward a bit an Angel appears to Joseph and tell him that there's some golden plates hidden in a hill Cumorah, NY and that it contained the story about a people who did a transatlantic ocean journey in 600 BC to the Americas and they were the ancestors of native americans. Joseph retrieves the plates and no one is allowed to see them but him and he sets out to translate them using a rock. This rock specifically:
(I'm not kidding)
So this becomes the book of mormon which details how there were these massive societies that settled in America (the promised land) and how Jesus came to America to visit after his death. Then these societies waged war with like 200k+ soldier armies until they killed each other off with swords and chariots (never mind there were no horses in the Americas at that point, or the wheel, let alone a chariot, nor was their steel or bronze).
Joseph goes onto set up fraudulent banks which get him kicked out of cities. He became a mason, after which he created a temple ceremony where you swear allegiance to the church and its prophets, has secret handshakes that are copies of the masonic rituals and you are sworn to wear underwear with masonic symbols on it. And they set out the plan of salvation which details the mormon path to salvation
He also prophesied that the garden of eden was in Jackson County, Missouri. He also quietly institutes polygamy, marrying some wives as young as 14, sisters, and other men's wives around 38 wives in total. This, surprisingly, made people angry. A group of angry ex-mormons took a printing press in Nauvoo, IL and printed pamphlet called the "Nauvoo Expositor" that outed his polygamy. Joseph ordered a group of faithful to destroy the printing press. Which they did. That, surprisingly, angered the sheriff that was already pissed that Joseph was already a thorn in his side due to the accusations of fraud and theft and he had Joseph and a few others arrested. An angry mob broke into the jail with guns. Joseph (who some how had a gun) shot two or three men before being killed.
After him Brigham Young took the reigns and ramped up the polygamy and moved the church to Utah so they could practice polygamy in peace. He also taught that black people bore the curse of cain on them which they got because they didn't fight valiantly in the pre-existence and banned them from the priesthood and thereby the temple. This meant that blacks were denied exaltation. They could get to heaven if they were good enough, but the best they could hope for was to serve someone more worthy in heaven. Also this also meant that black families would also be separated in heaven. The book of mormon teaches that if you were slothful or evil you would have darker skin and if you were righteous your skin would get lighter.
Fast forward to 1978 the church finally decides that maybe blacks are equal and should have the priesthood after all (there are some conspiracy theories on this being because the feds were about to launch an investigation against the church to deny their tax-exempt status due to their racial discrimination). They lift the ban, although the prophet after the one that lifted the ban famously wrote about how the civil rights movement was a communist plot.
Now the church is very anti-LGBT, funding most of Prop 8 and now actively engaged in trying to outlaw gay marriage in Mexico.
overall, I agree that mormons are very nice people outwardly. But I'd be lying if I didn't find that niceness to be skin deep. When I said, "Hey this sorta seems like bunk" I proceeded to lose all but 1 or 2 mormon friends. The church is inherently set up on exclusion (you can't go to a mormon wedding even if you're a mormon, you have to be a mormon in good standing meaning you keep all the rules, sustain the leadership, and pay 10% of everything you make to the church). With rules like no coffee, no tea, no smoking, no tattoos, no drugs and certainly no sex (the sin next to murder) they are very family focused and clean cut.
I agree that Under the Banner of Heaven is a pretty good intro. It's sorta crazy because you're taught that the church isn't like that at all. And certainly the modern church isn't but the early church was exactly that. I mean Brigham Young taught about "blood atonement" which taught that the only repentance for someone who didn't believe in him was to have their blood spilled, essentially justifying murder.
People say that Mormons are really happy, but what I've come to learn in my transition out is that what is good about the church isn't unique but what is unique about the church isn't good. Too much shaming, manipulation, etc etc.
