Christianity came a long way from the faith of an often-persecuted minority to one of the most influential religions in the Roman Empire by the time of Emperor Constantine. This eventually led to the recognition of Sunday worship around 321 AD, according to the Bible Timeline. The acceptance of Christianity in the empire took many years, and the repression by the authorities was brutal. However, its resilience saw to it that Christianity would be allowed (and even adopted) by the greater Roman population. The Edict of Toleration by Emperor Galerius issued in 311 AD paved the way for the acceptance of Christianity and ended the persecution of the people. Two years later, the Edict of Milan, agreed on and issued by Constantine I and Licinius, removed all the barriers that prevented the Christians from worshiping freely. The Edict of Milan also allowed the return of confiscated properties of Christians.
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This shift in policies concerning Christians was continued in 321 AD when Emperor Constantine issued a Sabbatical edict that was later compiled in the Codex Justinianus in 6th century AD. More importantly, the edict was the first recognition of Sunday as the day of rest and worship. In a message from Constantine to Elpidius, it states:


“Let all judges, the people of cities, and those employed in all trades, remain quiet on the Holy Day of Sunday. Persons residing in the country, however, can freely and lawfully proceed with the cultivation of the fields; as it frequently happens that the sowing of grain or the planting of vines cannot be deferred to a more suitable day, and by making concessions to Heaven the advantage of the time may be lost.”
It was further expanded during the time of the Emperors Valentinian, Theodosius, and Arcadius in the same codex which stated:
“The day sacred to the sun, to which the ancients very properly gave the name of Sunday, which returns after a certain period of revolution, must also be respected, so that there shall be no investigation of legal disputes on that day, either before arbitrators or judges, whether they have been appointed or voluntarily chosen.”
Ermatinger, James William. The Decline and Fall of the Roman Empire. Westport, Conn: Greenwood Press, 2004 http://legacy.fordham.edu/halsall/source/edict-milan.asp
http://droitromain.upmf-grenoble.fr/Anglica/CJ3_Scott.htm