Final answer:
The areas that remained mostly Roman Catholic in the United States are the Northeast, West, and Southwest regions, while the northern Midwest and Great Plains regions were mainly Lutheran.
Step-by-step explanation:
In the United States, the areas that remained mostly Roman Catholic are the Northeast, West, and Southwest regions. The dominance of Roman Catholics in these areas can be attributed to immigration from Roman Catholic countries in Europe, such as Italy, Ireland, and Poland, during the 19th and early 20th centuries. The strong Hispanic presence in the western United States and the Southwest also contributes to the Catholic dominance in these regions.
On the other hand, the areas that were mainly Lutheran are the northern Midwest and Great Plains regions, including Minnesota, the Dakotas, and surrounding areas. This is because German and Scandinavian Lutheran settlers migrated to these areas during the late 19th and early 20th centuries, and the Lutheran denominations have remained dominant in these regions to this day.