The Epistle of Barnabas is dated between A.D. 130-138, and was obviously not written by the Barnabas, but someone using that name. Nevertheless, it gives us a picture of the thought among early Christian leaders at that time. As Bacchiocchi notes, it gives us the first example of Sunday as being termed ‘the eigth day’, and it also depicts the tensions between Jews and Christians of that time, which led to changing the day of worship from Saturday to Sunday.

1. Further, then, it is written about the sabbath also in the Ten Words which God uttered to Moses face to face on Mount Sinai, “And treat the sabbath of the Lord as holy with clean hands and a pure heart.” 2. And in another place he says, “If my sons keep the Sabbath, I will let my mercy rest upon them.” 3. He mentions the sabbath at the beginning of the creation: “And in six days God made the works of his hands, and ended on the seventh day, and he rested on it and made it holy.” 4. Observe, children, what “he ended in six days” means. This is what it means, that in six thousand years the Lord will bring all things to an end, for a day with him means a thousand years. He himself bears me witness, for he says, “Behold, a day of the Lord will be like a thousand years.” Therefore, children, in six days, that is, in six thousand years, all things will be brought to an end. 5. “And he rested on the seventh day” means this: When is Son come and destroys the time of the lawless one, and judges the ungodly and changes the sun and the moon and stars, then he will rest well on the seventh day. 6. Further he says, “You shall treat it as holy, with clean hands and a pure heart.” If, then, anyone can now, by being pure in heart, treat as holy the day God declared holy, we are entirely deceived. 7. Observe that we will find true rest and treat it as holy only when we shall be able to do so having ourselves been made upright and had the promise been fulfilled, when there is no more disobedience, but all things have been made new by the Lord. Then we shall be able to treat it as holy, after we have first been made holy ourselves. 8. Further he says to them, “Your new moons and sabbaths I cannot endure.” You see what it means: it is not the one that I have made, on which, having brought everything to rest, I will make the beginning of an eighth day, that is, the eighth day with rejoicing, on which Jesus also arose from the dead, and having shown himself ascended to heaven (ch. 15).

– The Epistle of Barnabas, translation by E. Goodspeed (fn. 16), pp. 40-41.

As Bacchiocchi notes, the author of this Epistle argues that the rest of the sabbath days is not a physical rest but a rest at the end of days, the thousand years rest. He also argues that to keep the sabbath holy is impossible until this thousand years rest is commenced, and that, since God said “Your new moons and sabbaths I cannot endure”, this means that the current sabbath is unacceptable, and the real sabbath will be the one at the end of time.

This telling letter illustrates how early church fathers struggled to find scriptural support for changing the day of worship from Saturday to Sunday.