During His earthly ministry, Jesus taught that His followers would fast in the time between His ascension into Heaven and Second Advent (Matt. 9:15). We see this happening in places like Acts 13:2-3, where it’s mentioned that Christians were fasting and praying while worshiping the Lord. Unsurprisingly, after the New Testament era, regular fasting would quickly become a distinctive feature of the Christian church.
One of the earliest witnesses we have to this reality is the Didache, which was likely written before the turn of the 1st century. In chapter 8, readers are instructed not to fast on Tuesdays and Thursdays like the Jews, but instead on Wednesdays and Fridays. This is significant because these two days would quickly become “fasting days” across the entire Christian world, and the Orthodox East still observes them to this day. The fact that fasting on Wednesdays and Fridays is attested to so early, and the practice is so geographically widespread, makes it very likely that this custom came from one or more of the original apostles. It further suggests that early Christians perhaps had a more developed “liturgical cycle” than is often supposed.
And the Didache isn’t the only witness to a prescribed fast in the early church. Pagan critics of Christianity, like Caecilius (2nd century), mocked how Christians would hold “nightly meetings, solemn fasts, and [eat] inhuman meats” (Octavius Minucius Felix, 8). The reference to “nightly meetings” and “inhuman meats” is obviously about Sunday worship at the Eucharist, which we know from other pagans like Pliny the Elder (1st century). In his Letter to Trajan, Pliny mentions how Christians would worship Jesus on a “fixed day before dawn,” i.e. at night, but he corrects the misconception that Christians were cannibals (as Caecilius thought) by pointing out how the Eucharist is just “ordinary and innocent food.” Interestingly, Pliny says that part of the early Christians’ weekly worship consisted of them taking an “oath” to abstain from sin. The fact that Caecilius also mentions “solemn fasts” in the context of Christian worship, suggests that part of this oath may have been a promise to fast regularly. Perhaps this is why the Didache felt the need to address how fasting was to be done in the first place.
I would also argue that prescribed fasting is seen in the famous 1st century document, the Shepherd of Hermas. In 3.5.1, Hermas recounts how he was “fasting” on a certain mountain when the Shepherd came and asked him questions. The Shepherd asked why Hermas was fasting, and he replied that it was because of his “station,” which he further explains by saying that he’s simply “accustomed” to “observe” this fast. The Shepherd replies by pointing out how outward fasting is useless if it doesn’t coincide with an inward fast from evil, however, in 3.5.3 it’s revealed that this doesn’t mean that Hermas shouldn’t literally fast. Instead, Hermas is encouraged to keep observing this custom because, “If you do any good beyond what is commanded by God, you will gain for yourself more abundant glory.” Because Hermas has a “desire for doing good,” and “this fasting is very good,” Hermas is told to observe the fast “as I have commanded you,” i.e. both outwardly and inwardly.
Given the Didache and the Shepherd were both (probably) written in the 1st century, I think they serve as good evidence that the Christian church had a custom of prescribed fasting very early on. However, the Shepherd informs us that this fast wasn’t necessarily “binding,” the same way other moral commandments were, but it was simply a pious custom that was recommended for those who wanted to dive deeper into the spiritual life. The fact that pagan critics noticed this custom of fasting, and specifically tied it to the church’s liturgical life, further suggests that this practice was a distinctive feature of early Christian worship