Samuele Bacchiocchi, From Sabbath to Sunday (1977):

• Dr. Samuele Bacchiocchi, a Roman Catholic scholar who earned his doctorate at the Pontifical Gregorian University in Rome, thoroughly examines the origins of Sunday observance and the gradual abandonment of the seventh-day Sabbath within the early Church.

J.N.D. Kelly, Early Christian Doctrines (Harper & Row, Revised Edition, 1978):

• J.N.D. Kelly, an Oxford historian of early Christian thought, discusses in this influential work the beliefs and practices of the early Church.
  While the earliest Christians continued to recognize the seventh-day Sabbath’s biblical origins, Kelly’s survey shows that Sunday worship emerged as a custom rather than a scripturally commanded observance. He notes that no New Testament text explicitly transfers the sanctity of the Sabbath to Sunday, and early Christian writers themselves often acknowledged the biblical Sabbath while gradually embracing Sunday for various theological and social reasons.

The Catholic Encyclopedia (Vol. XIII, “Sunday”) (Robert Appleton Company, 1912):

• The Catholic Encyclopedia candidly acknowledges that the day of Christian worship was changed from Saturday to Sunday not by New Testament command, but by the authority of the Church. It notes that the early Church shifted from the Jewish Sabbath to Sunday observance as a matter of tradition. By admitting that the original Sabbath was Saturday, this source underscores that the biblical Sabbath predates the ecclesiastical decision to honor Sunday.

Cardinal James Gibbons, The Faith of Our Fathers (1917 edition):

• Cardinal Gibbons, a prominent Catholic leader, remarks that the Scriptures “enforce the religious observance of Saturday… a day which we never sanctify.” He acknowledges that the Bible itself does not authorize the observance of Sunday in place of the seventh day, confirming that the original practice was Saturday observance and that the later shift to Sunday stands on Church tradition rather than explicit biblical teaching.

Will Durant, The Story of Civilization: Caesar and Christ (Simon & Schuster, 1944):

• In this authoritative historical work, Durant, a renowned historian, notes that early Christians— many of whom were Jewish — initially kept the Sabbath. Over time, as Gentile converts increased and the Church differentiated itself from Judaism, Sunday gradually became the primary day of communal worship. Durant’s research confirms that this transition was not a direct result of biblical prescription but arose from evolving traditions within the early Christian communities.

The Schaff-Herzog Encyclopedia of Religious Knowledge (Funk & Wagnalls, 1908-1914):

• This respected reference work, edited by Philip Schaff and others, acknowledges that the New Testament does not abolish or replace the Sabbath with Sunday. Instead, it reveals how early Church writers, councils, and traditions influenced the Christian practice of Sunday observance over time. By recording these historical developments, the encyclopedia implicitly testifies that the scriptural Sabbath is the seventh day, and Sunday’s elevation came later through church authority rather than apostolic instruction.

A significant example regarding the Sabbath is found in Canon 29 of the Council of Laodicea (circa A.D. 363-364).

• This edict reveals that Christians were indeed observing the Sabbath on Saturday. The council’s prohibition is evidence that Sabbath-keeping among Christians remained common enough in the fourth century to warrant an official admonition against it.

Canon 29 of this council states:

• “Christians shall not Judaize and be idle on Saturday, but shall work on that day; but the Lord’s day [Sunday] they shall especially honor…”
  (Source: Canon 29 of the Council of Laodicea. English translations can be found in standard collections of canon law and various historical church documents.

The testimony of early church historians also corroborates the persistence of the seventh-day Sabbath.

• Socrates Scholasticus (5th century), a Greek church historian, shows one of his statements that while some major urban centers had shifted to Sunday-only worship, many Christian communications in the wider world continued to honor the Sabbath. He writes, “For although almost all churches throughout the world celebrate the sacred mysteries [the Eucharist] on the Sabbath of every week, yet the Christians of Alexandria and at Rome, on account of some ancient tradition, have ceased to do this.”

• Sozomen (5th century), another church historian, provides a similar account, stating:
  “The people of Constantinople, and almost everywhere, assemble together on the Sabbath, as well as on the first day of the week, which custom is never observed at Rome or at Alexandria.”

The survival of Sabbath observance can also be traced in regions distant from the Roman Empire’s ecclesiastical centers.

• Historical studies of the Ethiopian Orthodox Church attest that the church, whose roots may date back to apostolic times, has long honored both Saturday and Sunday as holy days, reflecting a tradition that clearly pre-dates later Roman decrees. This continuity of Sabbath-keeping in Ethiopia is documented by various scholars and researchers.

Historical research into the Celtic Church in the British Isles suggests that early Celtic Christians, prior to Roman influences, maintained Sabbath practices.

• Scholar Leslie Hardinge, in his work “The Celtic Church in Britain,” examines how these Christians observed Saturday as a day of rest and worship before being gradually aligned with Roman Sunday traditions.