In order to understand the Sunday Letter and its roots in the culture of the early Middle Ages, it will be helpful to examine the history of Sunday observance, insofar as this is possible from the limited witnesses of legislative, homiletic and narrative records. At first glance the language and the beliefs found in the Sunday Letter seem to be excessive; however, a careful study of the historical context will show that they would not necessarily have appeared so to a medieval audience. As the sanctity of the Lord's day grew in significance, so did the need to employ unconventional theology and forceful rhetoric in order to undergird the practice of complete Sunday rest from labour and attendance at the required church services. In the following, the issue of rest, rather than Sunday worship, will be the focus of the discussion, since that is the subject of the Sunday Letter.

The question would never have caused much concern had it not been a part of the one element of Old Testament law that was adopted wholesale into Christianity: the Ten Commandments. The two biblical listings of the decalogue formulate the commandment in quite different ways. They are reproduced in full here because of the recurrence of many of the following elements in medieval conceptions of Sunday observance: