Evidence suggests that Christians in the Egyptian city of Alexandria were the first to replace the Old Testament Sabbath with the first-day—that is, Sunday—worship. This may have begun as early as 115-120 AD. This information comes to us from the Epistle of Barnabas, arguably the oldest written documentation of Christians observing the first day of the week instead of the seventh day.
Although this epistle bears the name of Barnabas it was certainly not written by the Barnabas who appears in the New Testament book of Acts. It was probably written by someone in Alexandria, a city well known as a melting pot of religious ideas and classical philosophy.
The writer of the Epistle of Barnabas tends to interpret the Old Testament in a symbolic or metaphorical way. He condemns Judaism in general and the Sabbath in particular. He claims that he and his fellow believers are observing the eighth day of the week, the day after the Sabbath—obviously Sunday.
The Barnabas letter is the first evidence of Sunday being promoted as the Christian day of worship. But to find the real heart of the pro-Sunday movement we have to shift our focus from Alexandria westward to the heart of the Roman empire.