My argument is that Christianity is not defined solely by the Bible. That in fact, a Christian never even has to have read the Bible in order to be a Christian. That a Christian can still be a Christian even though they may pick and choose what they believe in the Bible, because it is not a demand that they follow the Bible to the T.
My evidence: When Christianity first began, there was not a such thing as a Bible. When Paul was preaching his message, the New Testament had just began loosely forming in the aspect that he was writing letters (which were not scripture). Even the OT canon was not closed until after Paul was dead. And during the time of Paul, different groups subscribed to different works of Hebrew scripture.
The Christian canon was not even fully closed until many centuries later. The canon had not even started to be put together until around a century later. This means the the first Christians did not have a Bible. They may have had works that they believed to be scripture, but various groups subscribed to different scripture. Even today, we see various scripture being held above others.
So obviously something else defined individuals who claimed to be Christians. And the same is true for today. A Bible does not define who a Christian is or what they believe. Just because it is in the Bible, does not mean that a Christian must follow it, or has to be defined by it. I think this has to be understood.
Too many people criticize Christians because they "pick and choose" what they want to believe. However, they have every right to do so as they are not defined by the Bible. Christianity evolved without the Bible. It began without the Bible. And for centuries, it existed without the Bible. In fact, for the vast majority of the history of Christianity, the vast majority of Christians have not had the chance to even read the Bible.