It might depend on the Baptist, but basically they think that since it is not explicitly spelled out in Scripture, it’s not true (or that there is not enough data to authoritatively rule on it.) Also, they might point to the passage that says “all have sinned.” (don’t have chapter and verse on hand). The problem is that “all” does not always mean “all without exception”. For instance there is a passage that says “all have died.” but there are exceptions Elijah and Enoch.
As far as it not being explicit in Scripture, that gets into bigger territory. Namely, if he is going to use explicitness as a rule, it is going to murder his own posistion on things like
Sola Scriptura (certainly not explicit, and I would argue not even implied in Scripture) and which books even belong in Scripture in the first place.
Start by digesting Marian teachings on Catholic Answers and perhaps this Dave Armstrong primer:
ic.net/~erasmus/RAZ153.HTM
But always remember that 99.98% of the time, a non-Catholic’s beef with Mary is not about Mary at all, but about the authority of the Catholic Church. If you can’t agree about that a discussion about Mary is only going to go so far.
Scott