Medieval Christians said the rosary as Catholics still do today: by praying five decades of Ave Marias (Hail Marys), each introduced by a Pater Noster (Our Father), while meditating on three series of five mysteries each.
Medieval Christians began the practice of saying the rosary
The system likely assumed that Medieval Christians began the practice, however the context does not say. The context only states that Medieval Christians did the practice, we don't know who first started it. 
( z1 begin :ARG0 ( z2 person :mod ( z3 religious-group :name ( z4 name :op "Christian" ) ) :mod ( z5 medieval ) ) :ARG1 ( z6 practice :ARG0 z2 :ARG1 ( z7 say :ARG0 z2 :ARG1 ( z8 rosary ) ) ) )