The answer
The Pope is given the title “His Holiness”, for he is the father of the church, and he has, through the grace of God, holiness in his personality. Because he is chosen by God for this service, he is considered to be the blessed of God. The Bishop, being the head and responsible for the priesthood, is granted the title “His Grace”. And the priest is given the title “Reverend”, which is also a derivative of holiness.
H.G. Bishop Moussa