As you can well imagine sabbath keepers have documented plenty of literary references to sabbath keeping throughout history. Most of them come from church historians who were not sabbath keepers. The following excerpt is cut and pasted from "Sabbath History", a chapter titled "Sabbath History Seventh Century to the Reformation." In it, it references several works that refer to sabbath keeping in the time period you mentioned:
START EXCERPT
Rome never succeeded in driving the Sabbath wholly from its dominions. We have reasons to believe that there have been Sabbathkeepers functioning in every century, some in the wildernesses, and some in and around the Alps. In their time they were known by such designations as Nazarenes, Cerinthians, Hypsistari, and later as Vaudois, Cathari, Toulousians, Albigenses, Petrobrusians, Passagii and Waldenses.
In speaking of the Waldenses generally, we know that they believed that the Romish Church was the Antichrist, spoken of in the Bible. Many of them were able to say a great part of both the Old and the New Scriptures by heart. Although considered dangerous heretics, Rainer Sacho, a Dominican, says of the Waldenses that they are one of the most ancient sects and that there was no country where they did not gain a footing. He admitted that they lived morally good lives and believed nothing concerning God which was not good (Dean Waddington's Church History, Chapter 22, Section 1).
Jones, in his history, in describing their confession of faith, says one of the members of the Waldenses stated that they 11 proffered the doctrine contained in the Old and New Testaments and comprehended in the Apostles' Creed, and admitted the sacraments instituted by Christ, and the ten commandments, etc.... They said they had received this doctrine from their ancestors, and that if they were in any error they were ready to receive instruction from the word of God . .." (Church History, p.355, 1837 ed.).
That the Cathari of the twelfth century observed the ancient Sabbath is certified by a Roman writer, as quoted by Doctor Allix:
"He lays down also as one of their opinions, that the law of Moses is to be kept according to the letter, and that the keeping of the Sabbath, circumcision, and other legal observations, ought to take place. They hold also that Christ the Son of God is not equal with the Father, and that the Father, Son, and Holy Ghost, these three persons, are not one God and one substance; and as a surplus to these their errors, they judge and condemn all the doctors of the Church, and universally the whole Roman church" (Ecclesiastical History, pp. 168, 169. The author was a French Protestant, born 1641).
Another group during the twelfth century was known in some parts of France and Italy as Passaginian. Of these Mosheim has written the following:
"Like the other sects already mentioned, they had the utmost aversion to the dominion and discipline of the church of Rome; but they were, at the same time, distinguished by two religious tenets, which were peculiar to themselves. The first was a notion that the observation of the law of Moses, in everything except the offering of sacrifices, was obligatory upon Christians, in consequence of which they circumcised their followers, abstained from those meats, the use of which was prohibited under the Mosaic economy, and celebrated the Jewish Sabbath" (Ecclesiastical History, Volume 2, p. 273, 1860 edition).
As to the charge of their circumcising, Benedict thought that this may not have been true but was said because they observed the "Jewish Sabbath."
There are at least three groups of eastern Christians that need to be considered in reference to this matter of Sabbathkeeping. Several historians indicate that for a long time the Abyssinian (Ethiopian) Christians were almost entirely shut out from the church of Europe. During the seventeenth century repeated and violent attempts were unsuccessfully made by the Jesuits, under the patronage of Portugal, to convert or subdue them. It is claimed by some, and by the Ethiopians themselves, that the gospel was brought to them by the Ethiopian eunuch (Acts 8), together with the teaching of the observance of the seventh-day Sabbath.
A. H. Newman says of them:
"Fasting periods are very numerous and about half of the days of the year, including the Jewish Sabbath and Sunday, are religiously observed. Indications of Jewish influence, besides Sabbath observance, are the practice of circumcision, and distinction between clean and unclean animals, etc." (A Manual of Church History, Volume 1, p. 646, 1933 edition).
Claudius Buchannan, D.D., speaks of the Armenians in the following manner:
"The history of the Armenian church is very interesting. Of all the Christians in Central Asia, they have preserved themselves most free from Mohammedan and Papal corruptions...."
"The Armenians in Hindoostan are our own subjects. They acknowledge our government in India, as they do that of Sophi in Persia, and they are entitled to our regard. They have preserved the Bible in its purity, and their doctrines are, as far as the author knows, the doctrines of the Bible. Besides, they maintain the solemn observance of Christian worship throughout our empire on the seventh day; and they have as many spires pointing to heaven among the Hindoos as ourselves. Are such people then entitled to no acknowledgement on our part, as fellow Christians? Are they forever to be ranked by us with the Jews, Mohammedans, and Hindoos?" (Researches in Asia, p. 206, et. seq.).
Arthur P. Stanley stated:
"The Chaldean Christians, called by their opponents Nestorians, are the most remote of these old 'Separatists.' . .. They trace their descent from the earliest of all Christian missions, the mission of Thaddaeus to Abgarus" (History of the Eastern Church, p. 91).
As to their beliefs about Sabbathkeeping, Coleman speaks:
"These eight festivals of our Lord they observer and we have many holy days and the Sabbath-day, on which we do not labour. ... The Sabbath-day we reckon far - far above the others. . . . Incense is burned on the Sabbath and feast days" (Ancient Christianity Exemplified, p. 573).
Early tradition attributes the founding of Christian communities in the south western part of India (Malabar) to the apostle Thomas. The first notice of this ancient people is to be found in Portuguese histories, according to Claudius Buchannon, D.D. When the Portuguese arrived they found upwards of a hundred Christian churches, whom they tried to win to the Romish faith by the power of the Inquisition. Dellon, who escaped the bloody tribunal, wrote an account of the workings thereof. His arrest occurred in 1673. Witness the following from Dellon's Account of the Inquisition at Goa, 1684:
"But when the period of the Auto da Fe approaches, the Proctor waits upon him and declares, that he is charged by a great number of witnesses, of having Judaized; which means, having conformed to the ceremonies of the Mosaic law, such as not eating pork, hare, fish without scales, etc., of having attended the solemnisation of the Sabbath, having eaten the Pascal Lamb, etc."
Sabbathkeeping in China is of ancient origins. Some historians maintain that Christianity was implanted in China not long after apostolic days. We have previously noted the Nestorian Christians, considered by many as heretics, were observers of the seventh-day Sabbath. Gieseler says of them:
"The Nestorians not only maintained themselves in Persia, where they enjoyed the exclusive favour of the king, but spread doctrines on all sides, carrying them into Arabia and India, and it is said, in -the year 636, even as far as China" (Church History, Second Period, Chapter 6).
Speaking of the Chinese Puritan Reformation, in modern times, under the Ti-Pings, one missionary in Shanghai wrote:
But the question naturally arises: How came they to adopt the seventh day of the week instead of the first, as their Sabbath, since all their instructions from Christians was by those who taught that the first day is the Sabbath? This was a mystery to all who learned of the fact. But when they took Nan-King, the Europeans had opportunity to visit them, they were told that it was first, because the Bible taught it, and second, because their ancestors observed it as a day of worship." These Chinese once observed the seventh day of the week as the Sabbath.
END EXCERPT
You are willing to name such blatant heretics in your support?
8. CERINTHIANS from Cerinthus and also Merinthians from Merinthus, saying that the world was created by angels, and that it is proper to be circumcised carnally, and to observe the other precepts of the law of this kind; that Jesus was only a man, and did not rise again, but asserting he will rise again. Also they gossip of a thousand future years after the resurrection in a terrestrial kingdom of Christ, according to carnal things of the belly and the delights of pleasure, wherefore they are also called Chiliasts.9. NAZOREANS, although they confess that the Son of God is the Christ, also however observe the old law, which Christians by apostolic tradition do not observe carnally, but know to understand spiritually. (St. Augustine, De Haeresibus)
Hypsistari - only reference I can find is your source.
Vadois were French Protestants.
The essential characteristic of the Catharist faith was Dualism, i.e. the belief in a good and an evil principle, of whom the former created the invisible and spiritual universe, while the latter was the author of the material world.
Toulousians - there is no reliable source which refers to them, so far as I can find.
The Albigenses asserted the co-existence of two mutually opposed principles, one good, the other evil. The former is the creator of the spiritual, the latter of the material world. ... The dualism of the Albigenses was also the basis of their moral teaching. Man, they taught, is a living contradiction. Hence, the liberation of the soul from its captivity in the body is the true end of our being. To attain this, suicide is commendable; it was customary among them in the form of the endura (starvation). The extinction of bodily life on the largest scale consistent with human existence is also a perfect aim. As generation propagates the slavery of the soul to the body, perpetual chastity should be practiced. Matrimonial intercourse is unlawful; concubinage, being of a less permanent nature, is preferable to marriage.
Peter of Bruys admitted the doctrinal authority of the Gospels in their literal interpretation; the other New Testament writings he probably considered valueless, as of doubtful apostolic origin. To the New Testament epistles he assigned only a subordinate place as not coming from Jesus Christ Himself. He rejected the Old Testament as well as the authority of the Fathers and of the Church.
Occupying a distinct place of their own were the pantheistic coteries of dissenters, the Amaurians and Ortlibenses, and perhaps other groups, like the Passagians and Speronistae, of which we know scarcely more than the names. ... The Passagii, or Passageni, a sect whose name is first mentioned in the acts of the synod of Verona, seem to have been unique in that they required the literal observance of the Mosaic law, including the Jewish Sabbath and circumcision. It is possible they are identical with the Circumcisi spoken of in the code of Frederick II. As late as 1267 and 1274 papal bulls call for the punishment of heretics who had gone back to Jewish rites, and the Passagii1037 may be referred to.
In their earliest period the Waldenses were not heretics, although the charge was made against them that they claimed to be "the only imitators of Christ." Closely as they and the Cathari were associated geographically and by the acts of councils, papal decrees, and in literary refutations of heresy, the Waldenses differ radically from the Cathari. They never adopted Manichaean elements. Nor did they repudiate the sacramental system of the established Church and invent strange rites of their own. They were also far removed from mysticism and have no connection with the German mystics as some of the other sectaries had. They were likewise not Protestants, for we seek in vain among them for a statement of the doctrine of justification by faith. It is possible, they held to the universal priesthood of believers. According to de Bourbon and others, they declared all good men to be priests. They placed the stress upon following the practice of the Apostles and obeying the teachings of the Sermon on the Mount, and they did not know the definition which Luther put on the word "justification." They approached more closely to an opinion now current among Protestants when they said, righteousness is found only in good men and good women.
They do not seem to have been sabbath keepers.
So basically you have a bunch of heretics; the only ones who kept the Sabbath also were implicated in other heretical practices, like circumcision.
The Ethiopian Orthodox Church exists today and it observes Sunday, and always has.
The Armenian Catholics are also, of course, perfectly orthodox on the question, as are the Armenian Orthodox.
We have previously noted the Nestorian Christians, considered by many as heretics, were observers of the seventh-day Sabbath.
If that was part of Nestorianism, it would have been condemned or brought up at the Council of Ephesus.
Your anonymous "missionary" is not credible:
You're posting was very interestign, and gives me worthwhile leads. However, I must note:
You're exerpt is from someone who uses the word, "Romish." The use of a non-standard perjorative doesn't speak well of someone's frame of references when they make unfounded assertions ("we have reason to believe...") Of the groups they identify, there is much better reason to quabble than what would be merely ad-hominem:
I think you'll certainly agree that the Cathari aren't Christian, given the denial of the divinity of Christ.
Of the Passiginian, they are plainly Judaizers, whether or not they circumcised, and from my limited understanding, were a short-lived an local phenomenon.
A.H. Newman regularly admits that the Ethiopes did keep Sunday. Their use of Saturday as a day of rest is not far from our concept of "weekend;" Copts certainly keep the day of Resurrection, Sunday, as most Holy.
As to Chaldean Catholics (A non-Latin rite of the Roman Church) and the Armenian (An Eastern Orthodox branch), I can attest from personal witness that they keep Sunday holy.
I am ignorant as to the ancient Chinese Christians, but remain dubious given the above facts.