But let's be honest, here. A good number of the things we do aren't found in Scripture either. The charismatic practice of raising one's hands in the air during worship isn't found in Scripture (unless one uses an odd interpretation of Saint Paul's statement "lift up holy hands"). Infant dedication isn't found in Scripture. My church celebrates Advent, Christmas, Holy Week, and Easter. None of those things are found in Scripture, with the exception of Easter. And according to Scripture, even Easter should be celebrated on the Hebrew day of 14 Nisan; it was church tradition that fixed Easter on a specific day according to the solar calendar. For that matter, prayer in any language other than Hebrew or Greek isn't found in Scripture. Greek was the church's official language until Saint Victor the Bishop of Rome (or Pope, for all the Catholics out there) began writing in his native language of Latin.
Baptists might not be Protestants, but the Protestant reformation has had an impact on us. Most of the influential figures in Baptist thought (at least at my church), such as Martin Luther and John Calvin, were Protestants. And chances are, they probably crossed themselves. I do not despise church tradition, which is why I'm not against this practice.
I think we need to remember that sola scriptura doesn't mean that Scripture is the only thing we ever read. It means that Scripture is the only writing that is infallible. Certainly we can base our worship around tradition. The very idea of holding one church service a week, with worship songs and a sermon, come from the tradition of the Jews. Just because something isn't found specifically in Scripture doesn't make it bad. If that were so, we ought to burn every copy of The Purpose Driven Life, which many evangelicals are so fond of. So I'm not sure I can see what anyone would find so objectionable about crossing oneself.
Sorry.
