Owl,
there was roman citizens, freemen (who had no rights against a roman citizen) and slaves(owned by all/any of the former 2) slaves were created by conquest and debt.(you owe you get sold) police actions can get you into slavery. Kids of slaves are slaves. You could buy yourself out of slavery.
During the second diaspora The romans determined to solve the jewish problem once and for all destroyed the temple, enslaved everyone they could catch (rather desultrily) and shipped them back to rome. In this group were early christians (remember that only Jews could become christians until the apostle paul came and changed all the rules, different rant) so the christians were indeed slaves in behavior and outlook the point of Andrew's comment.
thanx,
bill