In 325 A.D. Emperor Constantine was converted to Greek Christianity and then decreed it as the Universal  (which in Greek is Katholicos which became Catholic.) religion throughout the Empire. This is the one, holy, catholic, and apostolic Church that most all protestant Christian churches are part of. 

The Greek Christians converted the Roman Emperor into an Orthodox Greek Christian but Constantine by his decreeing it as the universal and official religion of the Roman Empire, became not a Greek Orthodox Christian but rather a Roman Catholic Christian.

The Roman priesthood brought their pagan statue worship (idol worship) when they merged with the Greek Orthodoxos (true-teaching) Church.

After obtaining possession of the Greek church's Hebrew manuscript called 'Matthews Logia", the Roman priesthood drew up their own version in Latin and "separated' it into four versions in order to conceal its teachings from the public, and is our present four-fold Gospel.

paraphrased from New Light in an Old Lantern, Joseph C. Bonner