Patheos is doing a series asking us to explain “Why I am a…” in 200 words or less.
I just wasted some of my word limit telling you that, so I'll get on with it.
Before I was a progressive Christian I was a conservative Evangelical Christian, and before I was a conservative Evangelical Christian I was a fairly nominal Catholic. So the answer is multifaceted.
I am a Christian partly because I was raised in a particular Christian context, and a broader one in which various forms of Christianity predominate.
I am a Christian today primarily because I had a life-changing born again experience in a Pentecostal Christian context.
I am a progressive Christian because my born-again fervor led me to study the Bible, theology, and related topics at university. Study led me to change my views. I am a progressive Christian because my study of the Bible, theology, and history, my use of reason, and the powerful religious experience I had, have led me here.
I must also add that I am an American Baptist for a number of reasons, one of which is that I can be all of the above quite naturally in that context.