I feel like except for 1.5 things, this is an alternate Earth in which the Mithrian religion of Roman times took hold and suppressed all other religions.

Things supporting my theory: 1) They are of the Mithraic religion 2) The armor that people wear is reminiscent of medieval armor. Pointy helmets, shoulder armor, etc. all has a medieval vibe to it.

Things opposed to my theory: 1) The name of the city is Boston. 2) The year 2145 is far enough out that maybe a new cult could take over the world. On the other hand, maybe they have a calendar that is based on something other than the birth of Christ, like the birth of King Saussatar of Mitanni (~1450 BCE), which would make 2145 an equivalent to our year 690 CE.