St. Gregory’s Parish in Lower Mills will begin the observation of its 150th anniversary this month with a 4 p.m. Mass on Sat., Oct. 20 — and a gala dinner to follow at Florian Hall. The sesquicentennial celebration is a milestone not only for the landmark church and for Catholics across Boston, but also for Dorchester residents of all faiths.

In 1863, St. Gregory’s became the first Catholic outpost in what was then the independent town of Dorchester. The parish was formed out of necessity, as immigrant laborers, many of them of Irish descent, flocked to this part of Dorchester to work in the chocolate mills and other industries that were flourishing along the Neponset River. Earlier, in 1847, Rev. Terence Fitzsimmons, pastor of Saint Peter and Paul Parish in South Boston, had secured land on Dorchester Avenue near where St. Gregory’s church now sits. According to a history of the parish on its website that was written by Rev. Michael Parise, who served at St. Gregory’s in the 1980s, Father Fitzsimmons had to engage in some subterfuge to execute the purchase, since anti-Catholic sentiment was at a fever pitch in Massachusetts in those days.

Seven years later, in 1854, the first St. Gregory’s church building, then under construction, was destroyed in a fire attributed to “Know-Nothing” nativists who had been terrorizing Irish Catholics with such attacks during the ante-bellum period.

Rev. Thomas McNulty was named the first pastor of St. Gregory’s in December 1862 as the first iteration of the magnificent Romanesque Revival church we know today neared completion. When it opened the next year, St. Gregory’s became a beacon for Catholic people in the town of Dorchester, the southern part of Boston, and surrounding communities. Most of the other parishes of Dorchester were later spawned from St. Gregory’s roots. The church was expanded in 1894, an ambitious effort that included a new red brick façade with the now-iconic dual towers that rise above Lower Mills village.

Even as demographic changes continue to swirl around it, St. Gregory’s remains the center of religious and civic life in Lower Mills. Its gymnasium hosts monthly civic meetings, flu clinics, summer camps and basketball clinics— open to people of all faiths. The grammar school across the street— now part of the Pope John Paul II Catholic Academy— remains a vibrant place of learning and faith development for young folks from Dorchester and Mattapan, in particular, as does Elizabeth Seton Academy, also housed on the St. Greg’s grounds. The church itself maintains a robust and devoted congregation of old and new residents from Boston and Milton. Since the 1980s, it has warmly welcomed the waves of new immigrants —from the Caribbean, in particular— into its pews.

The Oct. 20 celebration of St. Gregory’s foundation in the turbulent days of Civil War Boston will serve as a timely reminder of how far we have come in welcoming people of all colors and creeds into this community. It’s a history that we— as parishioners of St. Gregory’s— look forward to celebrating with all of our friends and neighbors who love Dorchester.

If you would like to buy tickets ($100) to attend the Oct. 20 dinner at Florian Hall (5:30 p.m.), go to the parish website at stgregoryparish.com. There is also a 10 a.m. breakfast planned for Sunday, Oct. 21 at the parish gymnasium across from the church. Tickets for the breakfast are $10 per person and $25 per family.