 The Diocese of Greensburg , to which Mount Saint Peter now belongs , was established on March 10 , 1951 , by Pope Pius XII . Records show that the first Mass in the area was celebrated in the Allegheny Mountains in 1749 by a French priest serving as chaplain to French troops . Demetrius Augustine Gallitzin , the second priest ordained in territories that became the United States , arrived in Loretto , Pennsylvania in 1799 . Father Boniface Wimmer came to Latrobe from Germany and established the first Benedictine presence in the region of Saint Vincent in 1846 . 
