It's hard to start talking about religion in the Greater Boston Area without starting with the fact that the two largest immigrant populations in the city are Italian and Irish. For this reason, the Catholic presence in the Boston is by far the majority in terms of numbers of followers. Almost half of all Metropolitan Boston residents identify in one way or another as Catholic.
For this reason, every town in the area has at least one Catholic Church community and some have many more than that. This being said, with the exception of communities on the North Shore which have more commonly followed their Catholic heritage, the majority of Boston Area Catholics are C+E Catholics (attend Christmas & Easter, plus life events listed for Lapsed), Lapsed Catholics (weddings, baptisms and funerals only) or Recovering Catholics (meaning that they were baptized in a Catholic church and haven't yet joined something else). In spite of this deep historical link to the Vatican, Boston is overwhelmingly neutral towards faith, often times shunning it as pre-enlightenment thought.
Judaism is also quite prominent in Boston as with other Northeast cities. Boston has a substantial and influential Jewish population and Greater Boston Jews are active in politics, philanthropy and of course traditional areas such as law, medicine and higher education. Some towns, most notably those west of the city, have tended to accumulate the majority of Boston Area Jewish populations, but some other towns such as Swampscott in the North have substantial Jewish populations as well. Heavily Jewish are Brookline, Newton, Lexington and Needham, all of which are somewhat west of Boston.
Strange to a city dominated by Catholics and Jews, Belmont, a nearby suburb of Boston is home to the Boston Mormon Temple, a monolith building overlooking Fresh Pond. While no town in Boston is particularly Mormon there must be many Mormons among us to substantiate such a huge home of worship.
Boston also has an Islamic population and the relatively upscale suburb of Wayland lays host to the Islamic Society of Boston which holds regular meetings with a local synagogue in that same neighborhood to show their solidarity with Jews in the area.
So, I have quietly avoided the obvious missing subject, which is that of Christian Churches or what is sometimes known as the Protestant movements such as Congregational, Methodist, Baptist and Evangelical. Leaving aside Congregational and Methodist sects, evangelical Christianity is frowned upon in Boston. Many ideas abound as to why this is and it most likely is linked to the heavy emphasis on continuity, tradition and loyalty which are deeply bred into Catholicism and Judaism. The city is highly educated, the state is highly liberal, and the communities (aside from those which are heavily Catholic on the North Shore) are generally welcome to alternative lifestyles. Boston's South End has a substantial gay population and gay families abound in most liberal suburbs. This includes the welcoming into faith communities, notably Unitarian and Jewish. Proselytizing just doesn't go well in Boston and people do not walk around with bibles or quote Jesus with each other. Still, some large churches such as Grace Chapel in Lexington or the Boston Church of Christ (Framingham) or more laid back First Baptist varieties do exist in Boston and some even go through spurts of growth. But nothing displaces the strength of the Catholic Church around Boston and everyone tends to tolerate everyone else's viewpoint as long as it doesn't encroach on theirs (or involve violence). Liberal and slightly crunchy Lexington has been the target several times of picketers by the extreme right wing Westboro Baptist Church for the town's welcoming attitude towards alternative family types and its clubs promoting tolerance in the High School.