Tim Cook answered
The oldest church in Montana is St Mary's Mission, a catholic church at the West end of 4th Street in Stevensville, in the south-west of Montana.
There has been a place of worship in this area of Stevensville since 1841, although the church as we know it now was not built until 1866, when Father Joseph Giorda, Father Ravalli and Brother William Claessens re-established the church about a mile south of Fort Owen.The oldest church in Montana
A chapel was built, along with a study, dining room, kitchen and a story and a half barn. Father Giorda made the this church the Jesuit mission headquarters for the Rocky Mountain province.
Can I visit the church?
Yes, but if you wish for the guided tour, please note these take place between April 15th and October 15th, Tuesday to Sunday, between 10am and 4pm. The tour includes a visit of the Mission complex, chapel and residence, dining area and kitchen, Father Ravalli's cabin and pharmacy, Chief Victor's cabin which houses a Salish museum and the old dove cote.
You will also have to view original hand carved furnishings, native American artifacts, historic documents and photos of olden times.
Placement of churches in Montana continues to be an issue, as we see here: