If I were to summarize why I abandoned Christianity in a few concise ideas, here's what I would point to: 
  1. Homosexuality.  Gay people aren't gay because they want to be gay; they're born that way, as are animals.  If God created people, he created gay people.  But God doesn't like homosexuality.  So either God made a mistake, or homosexuality disproves the existence of God.
  2. Evolution.  It's not a debate; we have fossils.  And unless God had the foresight to bury animal bones under specifically measured layers of dirt in order to trick future humans into believing something else, the creation story isn't true.  That means the Bible isn't completely true.
  3. New information.  In light of, or as a result of, the previous points, I realized I was at a disadvantage concerning new information.  There are only so many ways the Bible can be interpreted, so new information either agrees with it or contradicts it.  Homosexuality contradicts it.  Evolution contradicts it.  It's concerning how difficult it is for a person to change their mind about something.  I realize this cuts both ways, but at least I changed my mind once.
I realize these are ridiculously one-sided arguments (hello internet!), but they were essentially the focal point of my deconversion. #religion