The Bible confirmed my doubts about Mormonism.
I guess I was somewhat fortunate then.
I became a Christian in 1990, at the age of 27.
I had access to the Internet through my University's computer system, and I was part of a "Usenet" chess discussion forum. I put a Bible quote in my signature, and I was contacted by email by a Mormon on the chess forum, "Hi! I gather you're a Christian. I'm a Christian too, a Mormon Christian". Well, I knew almost nothing about Christianity until I became a Christian, and knew even less about Mormonism, but I loved reading Scripture, and had probably read the NT through at least twice by this point.
What started as an unsolicited email turned into a 10-year discussion between Doug and I, where we shared and discussed just about every aspect of our beliefs. We gave each other gifts. I sent him a modem he needed, he sent me a competition chess set, complete with chess clock. I sent him a copy of "Pilgrim's Progress", he sent me and LDS-KJV, and D&C and PoGP.
I had learned early on in my apologetics experience (I was very zealous about the gospel, and discussing it, and my first apologetics experience was on a local BBS network in discussions with a JW and an atheist (the latter with the nickname, "Mickey Finn"). I learned early on that if I wasn't familiar with a passage, I needed to read (at least) the surrounding paragraph (if not more).
I also picked up a copy of "Mormonism, Mama, and Me" by Thelma "Granny" Geer. At the time, I actually thought it was a "Pro-Mormon" book, but in the end it turned out to be very eye-opening for me.
After ten years of discussions, the experience only that much more convinced me that Mormonism was a cult. On Doug's side, he had started the exchange saying that every Mormon doctrine could be proven from the Bible. At the end of our discussions, he admitted to me that he was wrong about them all being taught in the Bible, but they were all taught in Mormon sources, so he still believed them. I don't know whether I convinced him of at least that much, or whether that was what he thought I wanted to hear.
I often say that of all the Mormons I've ever met, he's the only one I have any hope of becoming a Christian. Because he was always friendly, and willing to answer any question, and address any passage I brought up, instead of playing the games we see over and over in these forums.
There was one interesting anecdote. We are both chess enthusiasts, and so I suggested he try to find "ChessMaster 2000", an AI playing chess software. He found it at the local library, and loved it. Six months later, I discovered that he still had it installed in his computer, long after he returned the package to the library. I told him that he needed to delete it, since it was stealing. He had only "borrowed" it. He disagreed with me at first, but eventually told me he saw my view and deleted it.
Sadly, the discussions ended with a sad story. He was actually an employee at a library. And the last discussion which we had was about the necessity of water baptism. He tried to tell me that all the Christian denominations, including Catholicism, saw water baptism as "required". Well, I knew that this was completely false, and that even the RCC taught a "baptism of desire" (if you wanted to be baptized, but were unable before dying), and "baptism of blood" (for martyrs). So I asked him to support his claim, and he gave me a secondary source written by a Mormon. I shared with him primary sources from the churches in question, and he simply responded, "I don't accept them. I have all confidence that my author did his due diligence and studied it sufficiently". So this is a guy at a library, who believes (biased) secondary sources were more accurate than primary sources.