It is true that early Jewish-Christians continued to observe the Jewish Sabbath as a day of rest.  But it is not true, as Brown implies, that Constantine originated the shift to Sunday, nor is it true that the only (or even the main) reason for this was to utilize the day of the pagan sun god’s weekly tribute.  Early Christians referred to Sunday as the “Lord’s day,” because this was the day on which they believed he was raised from the dead.  There is evidence that Lord’s day (Sunday) celebrations already began to replace Sabbath observances (Saturday) during New Testament times (first century).  Even among those who continued to see Saturday as a “sabbath” in terms of a day of rest, there was often a celebration of the Eucharist on Sunday as well.  But the idea of a day of rest on Saturday already was being criticized and abandoned by Christians in the 2^nd century (Ignatius, Magn. 9:1-3; Letter of Barnabas 15).  Some Christian leaders supported the idea of a day of rest, but urged that it be observed on Sunday, to coincide with the celebration of the Lord’s day and the Eucharist.  At any rate, it was common long before Constantine’s time to celebrate the Eucharist on the Lord’s day (Sunday), and there would have been virtually no one in Christianity by Constantine’s time who was still observing the Sabbath as a day of rest on Saturday.  When Constantine signed a law on March 3, 321 C.E. that required rest from work on Sunday, he was not creating a “new” practice for Christians but simply legalizing and mandating a practice that had long been conducted by Christians.