"May the apostates have no hope. May the dominion of wickedness be speedily uprooted in our days." (The Blessing on the Heretics)
After the Temple was destroyed in 70 AD, the synagogue became the new center of Jewish religious life. Christians commonly worshiped alongside their Jewish brothers. This troubled some synagogue leaders, who wanted unity and uniformity within their congregations. In order to identify and expel the nonconformists they added a new element to the prayers or blessings known as the Eighteen Benedictions that were part of the synagogue service.
Any man in attendance could be called upon to recite these benedictions, but a Christian man would certainly not call down this curse upon himself. He would have to omit or attempt to alter his recitation of this "blessing" and, by doing so, identify himself as a "heretic." Then the congregation could expel him for cause.
The Blessing on the Heretics is evidence of Christian Sabbath observance seven decades after the crucifixion of Christ.
Some people suppose that only Jewish converts to Christianity kept the Sabbath while Gentile converts did not. If this were the case, however, we would expect some evidence of tension and debate over the issue. But there is simply no clear historical data suggesting that such a debate existed until well after the end of the first century.