This is a topic which comes up occasionally here--one group converts to a certain religion and because of this, they have a distinctness from society which helps preserves many elements of their culture and most noticebly their language despite the world around them changing immensely. To me it seems fascinating since it's a way to preserve the language and culture of peoples of Antiquity into later times.
The ultimate and most prominent example of this is the Jews, who preserved a culture that emerged out of the Iron Age Near East and formed it into an extremely prominent religion. Their linguistic heritage, the Hebrew language, survived longer than all other Canaanite languages (aside from Punic, which only survived because it retreated to North Africa) AND was successfully resurrected in the modern age. The language the Jews shifted to, the Jewish dialects of Aramaic, were still widely spoken despite the Arab conquests until the foundation of Israel in the 20th century. That's quite the impressive accomplishments of the Jewish people.
As we know, there's others aside from the Jews. Many Christians of the Middle East preserve Aramaic dialects long after they were subsumed by Arabic. The Copts spoke Coptic until the Early Modern age and still use it as a liturgical language, like Hebrew was for centuries--other Coptic traditions preserve pre-Islamic Egypt. The Mandaeans preserve a unique Aramaic-derived language into the modern age, and even a unique Aramaic-derived alphabet. Zoroastrians in modern Iran have their own unique dialect of the Persian language. Karaites and Samaritans, both Jewish-derived, preserve unique languages of their own. The Amish and some Mennonites hold on to a particular German dialect, the Pennsylvania German language. In the future, the Mormons will probably have their own variant of English. Examples clearly abound.
But I bring up the point of preservation again. Is the ability of religious traditions to preserve languages any way overrated? A while back there was a short TL here involving Sumerian speakers who had a unique interpretation of Christianity/Islam (IIRC) in the modern age. That's an extreme example, but slightly more plausibly, is it really a case that if the right people in, say, cultures which spoke Tocharian and Anatolian languages, converted to Christianity or an otherwise unique religion (like a unique/heretical interpretation of a religion), those languages might have survived along with elements of the cultures which spoke them?
By extension, if Europe and the Middle East are fractured in terms of the religious landscape, with no clear lines drawn, would we have many languages of Antiquity surviving, like Continental Celtic languages (Noric, Galatian, etc.), East Germanic languages, all the aforementioned languages (Tocharian, Punic, Anatolian languages like Pisidian, Isaurian, and Cappadocian which likely survived into CE times), unique dialects of Greek (Cyrenaican), unique Iranian languages (Sogdian, Bactrian, etc.), "Paleo-Balkan" languages, etc. That's a substantial list, but is this phenomena of an "ethnoreligious group" easily plausible to save even a few of these languages as well as elements of those cultures?