Apparently that depends on which flavor of Baptist theology one adheres to. The Southern Baptists of today are mostly Calvinists. And Baptists seem to regard Roger Williams as one of their own.
“The Southern Baptists of today are mostly Calvinists.”
Not even close - even 20% would be a stretch. I would say, however, that with Al Mohler (Calvinist) as President of Southern Theological Seminary that the percentage of calvinist Baptists will increase. As a whole, though, the SBC is overwhelmingly arminian.
That may think that they are Calvinists, but they are not Calvinists. They do not practice infant baptism, and they will rebaptise others, exactly as Mennonites do.
Their only difference is pacifism. Mennonites are pacifist and Baptists are not.