Christians who gathered in Rome were often thought to be engaged in secret rites. These rites made them highly suspicious to their non-believing neighbors. One rite in particular raised eyebrows: the Lord's Supper. In this rite, bread and wine were used in remembrance of Christ's sacrifice. When the Romans heard that Christians "ate flesh" and "drank blood," and while doing this would say Hoc est corpus [meum], they thought that these Christians were performing witchcraft. Eventually, Hoc est corpus was shortened to hocus-pocus -- a phrase we associate today with magicians.
Now you know!