the use of the title for a secular priest of father in the USA came with the arrival of irish immigrants . most of the priests that they encountered in their home land were members of religious orders. within religious orders the title father was given to priests to distinguish them from brothers. in the older orders all but the superior were addressed as brother. in time , the priests were called father and the seminarians frater . this usage distinguished those who had a vote in the order's councils from the brothers who were addressed brother in the local language.

prior to the larger irish immigrations the secular priests ( those without vows , mostly diocesan ) were addressed as mister and the bishop as doctor. in italy spain etc the secular clergy were addressed as don. the title of respect but not an impressive one. 

up until the sixties the listing of those in processions separated the religious and secular in print using the title of father and the reverends .