With Rosh HaShanah and the beginning of the high holidays only a few days away, I thought I would post a blog reviewing some of my material on this from my book, God-Fearers. It should be no shock to anyone that the early believers—both Jew and Gentile—celebrated the festivals of Israel even after coming to Messiah. This included the celebration of Rosh HaShanah and fasting on Yom Kippur.
Yom Kippur is to Judaism what Christmas is to Christianity; just as many Christians go to church on Christmas even if they never enter a church throughout the rest of the year, so too, most Jews fast on Yom Kippur and attend synagogue services. It is a cultural and spiritual landmark. Not surprisingly then, we find a passing reference to this fast in the book of Acts:
Since much time had passed, and the voyage was now dangerous because even the Fast was already over, Paul advised them, saying, “Sirs, I perceive that the voyage will be with injury and much loss, not only of the cargo and the ship, but also of our lives.” (Acts 27:9-10)
“The Fast” referenced here is Yom Kippur. Daniel StÃ¶kl Ben Ezra assumes that Luke would not have used the term “the Fast” as a calendaric reference unless he himself was keeping the fast and assumed that his readers were as well:
I cannot help but draw the conclusion that Luke himself and his implied readers observed Yom Kippur. Why else would Luke use a “Jewish calendaric reference for a secular problem?” He clearly presumes that his readers will understand what he is referring to. 
Scholars speculate that the readers of Acts were Gentiles as Luke himself was. In order for Luke’s readers to understand such a passing reference, they must have been observing Yom Kippur. We can find indirect evidence in the book of Revelation. The book of Revelation is packed with allusions to the rituals and themes of the high holidays. The apocalyptic imagery—the day of judgment, the books of judgment, the blast of trumpets, the Temple scenes, and so forth—is borrowed directly from the traditional observance of Rosh HaShanah and Yom Kippur. John addressed the book of Revelation to seven communities in Asia Minor, constituted predominantly of Gentiles. If those communities were not celebrating the high holidays along with the Jewish community, they would have been ill prepared to decipher the book of Revelation, just as the church is today. Records of Gentile believers celebrating Yom Kippur appear in later church literature. The late second-century Epistle to Diognetus rails against Christians who observe Jewish laws such as “the Fast.”  Origen mentions Christians fasting on Yom Kippur:
Whence also we must say something now to those who think that in virtue of the commandments of the Law they must practice the fast of the Jews. (Homily on Leviticus 12:2 [Barkley])
He mentions another case, again involving Caesarean Christians, in his Homilies on Jeremiah, proving that this is not an isolated instance; there must have been at least several groups of Gentile Christians in the third century still celebrating this major holy day of Judaism.  In the late fourth century, John Chrysostom was still denouncing those who “join the Jews in keeping their festivals and observing their fasts.” 
Further witness to this phenomenon can be found in the fifth-century medieval church practice of the Fast of the Seventh Month. This fast formed part of the Ember Days and was one of the most solemn days of the church’s liturgical year. Scholars see this fast as the result of the Christianization of Yom Kippur.  In other words, because many Christian Gentiles were celebrating Yom Kippur, as the church began to split from Judaism, Christianity slowly transformed it into a solely Christian fast in the month of September. This is similar to the transformation of Passover into Easter. The fifth-century theologian and pope Leo the Great wrote:
We proclaim the holy Fast of the Seventh Month, dearly-beloved, for the exercise of common devotions, confidently inciting you with fatherly exhortations to make Christian by your observance that which was formerly Jewish. (Sermon 90:1)
In this we once again see that many Gentile believers celebrated Yom Kippur, so much so that it remained even after the parting of the ways began to take place between Judaism and Christianity.
- Daniel StÃ¶kl Ben Ezra, “Christians Observing ‘Jewish’ Festivals of Autumn,” in The Image of Judaeo-Christians in Ancient Jewish and Christian Literature (eds. Peter J. Tomson and Doris Lambers-Petry; Tubingen, Germany: Moher Siebeck, 2003), 62.
- Epistle to Diognetus 4.
- Ben Ezra, “Christians Observing ‘Jewish’ Festivals of Autumn,” 69.
- Against the Jews 1:5.
- Daniel StÃ¶kl Ben Ezra, “Whose Fast is It? The Ember Day of September and Yom Kippur,” in The Ways That Never Parted (eds. Adam H. Becker and Annette Yoshiko Reed; Tubingen, Germany: Moher Siebeck, 2003), 259-282.