text: In England in the Midde-Ages I am given to understand that Religious Order priests e.g.Friars, who looked after the spiritual needs of people through preaching, hearing confessions and spiritual guidance were addressed as Father and secular priests as Mr. 
  In the Divine Office when asking the Hebdomadarian for a blessing the formula is, 'Jube, Domne, benedicere," (Pray Sir, a blessing), not Pater (Father). 'Domne' being a contraction of 'Domine' (Lord) to distinguish it from Our Lord. Some monks and Canons Regular are addressed as Dom and some abbesses and nuns as Dame. Italian priests are often addressed as 'Don'.
  As the Religious Orders were suppressed at the Reformation all the clergy left were secular and continued to bear the title Rev Mr to this day.
  French priests are often addressed as M'sieur l'Abbé which derives from the Hebrew or Aramaic for 'father'. 
  I also understand that deacons in the Eastern Churches are also addressed as Father; in the West it is 'Deacon' or Rev Mr.