St. Peter's Pro-Cathedral
It served as the pro-cathedral of the Archdiocese of Baltimore until an official cathedral , the Basilica of the National Shrine of the Assumption of the Blessed Virgin Mary , was built in 1821 to alleviate overcrowding at St . 
<triplet> Basilica of the National Shrine of the Assumption of the Blessed Virgin Mary <subj> 1821 <obj> inception