calling the pope "holy father" (a title reserved for God the Father ONLY) QUOTE]
"The origin of the Roman Catholic title, according to a catholiceducation.org article by Jean-Claude Perisset, was "born" during the Investiture Controversy.
http://www.catholiceducation.org/articles/apologetics/ap0038.html
The qualifier “holy” underlines the spiritual dimension of this fatherhood exercised in the name of God; and we have already said that it does not imply a moral judgment on the person of the Pope. The expression “Holy Father” was born in the time of the controversy over lay investiture, and it seemed normal that in its becoming common usage in the acts of the chancery, the Roman Curia had then wished to underscore the spiritual and supernatural level of the mission of the Pope by adding the adjective “holy” — to defend implicitly the superiority of papal power over imperial power.
Catholics and Orthodox Christians call priests "Father" based on St. Paul's theology of the spiritual fatherhood of priests: "I do not write this to make you ashamed, but to admonish you as my beloved children. For though you have countless guides in Christ, you do not have many fathers. For I became your father in Christ Jesus through the gospel" (1 Cor. 4:14-15).
The Pope is referred to as "the Holy Father" by Catholics NOT because he is any holier than anyone else on earth but because of his mission of spiritual fatherhood over the whole Catholic Church. The "Holy" part emphasizes that this is a spiritual relationship.
Protestant Christians do not follow this tradition."