Among Baptists there is pretty much a consensus on these things because they learned from other Baptists. Their Baptist parents told them Bible stories when they were children (and never once was Mary mentioned outside the Christmas story), they sat in Baptist pews and listened to Baptist preaching from the time they were old enough to listen, their Baptist Sunday School teachers taught them the truths of scripture from a Baptist perspective, etc. etc.
But there are other perspectives from which to learn about Christianity. I will give you two verses, by way of example, just to point out how different traditions view the same words.
John 3:5. Baptists and other evengelicals view this as referring to and contrasting childbirth with a spiritual rebirth. Those of the Catholic tradition view it as a reference to baptism. Both sides are so sure of their position that they actually ridicule the other for its ignorance.
Hebrews 12:1. Baptists and other evangelicals consider these witnesses are witnesses to us. Those of the Catholic tradition consider that they are witnesses of us. Again, both sides are equally tenacious in their beliefs.
One more example is the perpetual virginity of Mary. You can't prove this either way by using scripture but you can sure make some strong arguments on either side. In all of these things, we read scripture with the lens of our tradition. If you deny it, you are either hopelessly ignorant or hopelessly dishonest.
Surely you realize I was being sarcastic when I talked about a consensus on those issues. Even a cursory reading of this message board would reveal nothing even close to a consensus among baptists on those issues.
You say a baptist believes something because he/she grew up listening to storied from parents. What an ignorant statement! Really, you didn't think that one through, did you?
First, you assume baptists never mention Mary outside the so-called Christmas story. What an assumption. I would be interested to hear how in the world you plan on proving such an assumption is true.
Secondly, you totally discount the possibility that people actually convert from one demonination to another. Some people even convert from not being Christians to being Christians. Gasp! I wonder how they learn anything, seeing their parents surely didn't raise them up telling them baptist-leaning bible stories and filling their heads with all sorts of anti-catholic propaganda.
Finally, you make another grave error in assuming that baptists interpret John 3:5 the same way. There are several different ideas. I take it to be speaking wholly of regeneration, but I know some take it as natural and spiritual childbirth. The point is, there isn't a consensus.
You assume the baptists work like the RCC does. They do not. The truth is, were you to talk into a different baptist church every week, you could find a preacher with widely different views on a variety of subjects. One might be a calvinist, while the next is free will salvation. One might believe in limited atonement while the next believes in universal atonement.