Before you were a Calvinist, what were you?
[Disclaimer: In none of the following is it in my heart to parenthetically add "as opposed to you". This is just my little story as plainly as I can tell it. :)]
I suppose I was something like an Arminian Protestant, although I had never heard the term before at the time. I believed it was totally up to me to decide to choose Christ or not, and that I would be cooperating with God throughout the rest of my life in trying not to sin as much, doing good deeds more than otherwise, and going to church. While I believed in the plain meaning of OSAS, the general plan was that I would keep God happy (thus proving to myself that my faith was true), and He would let me into Heaven.
Then, I studied with a mentor for several years in private Bible study. Interestingly, he never once mentioned the names of Calvin or Luther, and never mentioned "Reformed" theology. (I hadn't even heard of Reformed theology until FR.) Anyway, there were two things about this Bible study which really changed me. One was his focus on Christ himself. He harped all the time that EVERYTHING is about Christ, and nothing is about us. He did it so many times that I OFTEN wondered to myself "yeah, yeah, I get it, I heard you the first time". I didn't get it. It wasn't until later, so now I understand why he did it.
This really got me to start to change my thinking about my whole role in ....... "everything". At some point I had completely re-prioritized my outlook away from men (me) and toward Christ. I think it was at that time that I probably became a Calvinist, even though I still had no idea what that term meant. When I later learned about what Reformed theology was, here on FR, the concepts were absolutely simple to absorb.
The other area that changed me was his focus on scripture. He made me memorize tons of it. We made a game of having "verse-offs" every week to see who had done his homework. :) At the time I just thought that he knew best so I went along with it. I didn't understand why until later. I think in part he was showing me how easy it is for a person to fall back into his self-absorbed ways. Knowing the scriptures in your heart will always be a strong check against that. I see that same focus in the Reformers.
Sorry if this is more than you wanted you know. It just brought back a lot of memories so I went with it. :)