He studied in Constantinople and beginning in 1822 in Rome at the Seminary of S. Pietro in Vaticano and the Pontifical Urban College for the Propagation of the Faith, where he earned a doctorate in theology in 1832.

[3] The election was made according to a decree of the Congregation for the Propagation of the Faith dated 9 May 1842, which was approved by Pope Gregory XVI on 23 May 1842.

He immediately set about planning for an order of religious women, eventually founded as the Armenian Sisters of the Immaculate Conception in 1847.

The bull also amended the rules for the election of bishops to remove any role for the clergy and laity.

[6][7] Hossoun participated in the First Vatican Council (1869-1870) where he was member of the commission on dogma and supported the definition of papal infallibility.

Pope Pius IX excommunicated the schismatics on 14 June 1872 and the responded by expelling Hossoun in July and he took refuge in Rome.

[3] Under pressure from France, the Turkish Sultan agreed in 1874 to recognize both churches, one loyal to Rome and one not, which allowed Hossoun to return to Constantinople in 1876.