There are diocese older than Baltimore, for instance New Orleans, but they were not part of the U S at the time of independence. Baltimore was the first diocese established in the United States.
The Basilica of the Assumption of the Blessed Virgin Mary, is not the oldest cathedral in the United States, but it was the first cathedral built in the United States. At one time, the Archdiocese of Baltimore included the entire U S. Boston, NY, Philadelphia, etc. were all established as suffergan diocese, and later elevated. The Baltimore Catechism is so-named because it was adopted by the Council of Baltimore, the largest assemblage of Catholic bishops outside of Rome since the Council of Trent.