Kallas was ordained a priest on 13 August 1958 by the White Fathers, and joined to the Melkite congregation Missionary Society of St. Paul, where he was Superior General from 1987 to 1993.
The ordination took place on 19 February 2000 and was performed by Jean Mansour SMSP, Titular Archbishop "pro hac vice" of Apamea in Syria of Greek Melkites and Auxiliary Bishop of Antioch.
The archbishop Kallas encouraged inter-religious dialogue between Islam and the Christians, and participated in many events.
[2] Basically, Kallas underlined the serious and historical development in the Eastern Churches and their apostolic understanding within the meaning of Apostle Paul of Tarsus.
Under the direction of Gregory III Laham, he participated at the Synod of the Melkite Greek Catholic bishops.