After the Resurrection, without losing their Jewish roots, Christians moved their day of worship to Sunday in honor of the Resurrection - and launched a movement that would span the globe. They spread His teachings, worked His miracles, served the needy and above all else, proclaimed the Gospel. All with the very real threat of death and martyrdom hanging over their heads. They would not have died and suffered for what they knew to be a lie.
I find it intriguing that you admit that
Christians moved the day. In this, you are absolutely correct. The question is however, by worshiping and gathering together on another day other than Sabbath, were they defiling Sabbath? Not necessarily. They may have still recognized Sabbath as a sacred day and ceased from work to maintain that sacredness. Allow me to offer you a little history lesson.
Christians did meet in the synagogues along with their Jewish brothers and sisters in the very early years. They, of course, used these opportunities to teach Christ and Him crucified, and resurrected, as you mentioned. The Jewish rabbis however naturally did not take too kindly to this. Persecution (remember Saul/Paul) against the early Christians came from solely Jewish religionists. The Christians of course, as persecution arose, became less bold, or more circumspect and careful in their proclamation of the new faith...to 'out' the Christian believers from the synagogues, the Rabbis devised new prayers that blasphemed Christ. Anyone not joining in was exposed. Christians then began to gather in homes and the riversides etc (remember Lydia). Add to this was the malice against the Jews by Rome which became much more intense as Jewish uprisings by zealots became a thorn in the side of Rome from the second century on. The Sabbath was a uniquely Jewish day, and Christians, not wanting to be associated with or be mistakenly recognized as Jews, compromised as a safety mechanism.Then an additional factor was Mithraism. The worship of Mithra became popular with the Romans, particularly in the military. They worshiped their sun god, yes you guessed it, on the first day of the week, and named it Sunday. Christians, as you would know, were persecuted then by pagan Rome (Satan cared not who his tools were, so long as he had vengeance against Christ) and this continued until the 4th century when Constantine made Christianity, or at least his version of what he wanted Christianity to be, official. In the 4th century, Sunday then became a civil law under the aegis of Constantine, the first emperor to devise such a civil Sunday law. The Roman and Alexandrian churches, both in the 4th century and on having become infected with Greek mysticism and Gnosticism, found the Roman civil law quite convenient and adopted it into canon law.
These factors combined all led to a great falling away from the truth. Although history reveals that some Christians observed both days even into the 5th century, while pagan Rome had ceased to persecute Christians, Papal Rome took up the mantle using pagan Roman emperors as their weapon of choice...until the barbarians took over from pagan Rome in the west and the Popes 'converted' them to be used as their army against Christians that refused to submit to Papal authority. Clovis, king of the Franks was the first of these, although, through manipulation and intrigue and influence, the early popes managed to create wars against 'heretics' by such as the Ostrogoths.
Two principle 'heretical' beliefs incited these wars and persecutions. The first was what we today know as 'Arianism'. Three entire nations were destroyed because of their beliefs on the Godhead (the 3 horns uprooted Romans 7) although it must be admitted the only source as to what those Arians actually come to us from their enemies, as Rome destroyed all of Arius's writings. There is one portion of his writings remaining, but as it comes from that same enemy, there is some doubt as to its veracity. The second cause for persecution of course was Sabbath observance. Rome hated the Sabbath, with a vengeance, and they still do. It is at Rome's instigation that Sunday laws became entrenched anywhere the Roman clerics had influence...this of course was extended through Protestantism as inheritors of this false doctrine, as evidenced the blue laws of early America. Many very large communities of Christian faith and practice observed the Sabbath until such time as Rome forced them to do othewise. The Waldenses, the Celtic church in Britain, and the Assyrian church of the east were such.
An interesting history indeed...and when one considers the paucity of Biblical evidence for Sunday sacredness and the bloody means by which Sunday was promoted and enforced, it should bring about a deep sense of embarrassment for Sunday observers to attempt to justify it today.