I think the reason why the church changed from saturday worship to sunday was to seperate themselves from the Jews, due to the Jews rejecting Jesus.
But now that the Jews are beginning to accept Jesus, the church is also now accepting the sabbath day as the true day of worship, so that both Jew and gentile accept Jesus and accept sabbath day.
A little problematic, seeing that all the first "Christians" were Jews, who never gave up the Sabbath. Nor did Christians themselves until a much later date.
In the 2nd Century
"The primitive Christians did keep the Sabbath of the Jews;..therefore the Christians for
a long time together, did keep their conventions on the Sabbath, in which some portion
of the Law were read: and this continued till the time of the Laodicean council." The
Whole Works of Jeremey Taylor, Vol. IX, p416 (R. Heber's Edition, Vol.XII, p.416)
"The gentile Christians observed also the Sabbath." Gieseler's Church History, Vol.1,
ch.2, par.30, p.93.
"The primitive Christians had a great veneration for the Sabbath, and did spend the day
in devotion and sermons. And it is not to be doubted but they derived this practice
from the Apostles themselves, as appears by several scriptures to that purpose."
Dialogues on the Lord's Day. p.189. London: 1701. By Dr. T. H. Morer.(church of
"The Sabbath was a strong tie which united them with the life of the whole people, and
by keeping the Sabbath holy they followed not only the example but the command of
Jesus." Geschichte des Sonntags, pp.13,14.
"It is certain that the ancient Sabbath did remain and was observed (together with the
celebration of the Lord's day by the Christians of the East Church) three hundred years
after the Saviour's death." A learned Treatise of the Sabbath, p.77.
In the 3rd Century.
"The seventh-day Sabbath was.. solemnised by Christ, the Apostles, and primitive
Christians, till the Laodicean Council did in a manner quite abolish the observation of
it." Dissertation on the Lord's Day, pp.33,34,44.
"As early as A.D.225 their existed large bishoprics or conferences of the church of the
East (Sabbath-keeping) stretching from Palestine to India." Mingana, Early Spread of
Christianity. Vol.10, p.460.
"Thou shalt observe the Sabbath, on account of Him who ceased from His work of
creation, but ceased not from His work of providence: it is a rest for meditation of the
Law, not for idleness of the hands." The Anti-Nicene Fathers, Vol.7, p 413, From
Constitutions of the Holy Apostles, A document of the 3rd and 4th centuries.
"After the festival of the unceasing sacrifice [the crucifixion] is put the second festival of
the Sabbath, and is fitting for whoever is righteous among the saints to keep also the
festival of the Sabbath. There remaineth therefore a Sabbatismus, that is a keeping of
the Sabbath, to the people of God [Heb 4:9]" Homily on Numbers 23, par.4, in
Migne, Patrologia Greaca, Vol. 12, cols.749,750.
In the 4th Century.
"It was the practice generally of the Easterne Churches; and some churches of the
west..For in the church of Millaine [Milan];.. it seemes the Saturday was held in farre
esteeme ..Not that the Easterne churches, or any of the rest which observed that day,
were inclined to Iudaisme [Judaism]; but that they came together on the Sabbath day,
to worship Iesus [Jesus] Christ the Lord of the Sabbath." History of the Sabbath
(original Spelling retained) Part 2, par. 5, pp. 73,74, London: 1636, Dr. Heylyn.
"The ancient Christians were very careful in the observation of Saturday, or the seventh
day..It is plain that all the Oriental churches, and the greatest part of the world,
observed the Sabbath as a festival..Athanasius likewise tells us that they held religious
assemblies on the Sabbath, not because they were infected with Judaism, but to
worship Jesus, the Lord of the Sabbath, Epiphanius says the same." Antiquities of the Christian Church, Vol. II, Book XX, chap. 3, Sec. 1, 66.1137, 1138
"From the apostles' time until the council of Laodicea, which was about the year 364,
the holy observation of the Jew's Sabbath continued, as may be proved out of many
authors: yea, notwithstanding the decree of the council against it. Sunday a Sabbath,
John Ley, p.163 London 1640.
"Ambrose, the celebrated bishop of Milan, said that when he was in Milan he observed
Saturday, but when in Rome observed Sunday. This gave rise to the proverb 'When
you are in Rome, do as Rome does,' " Heylyn, The History of the Sabbath, 1613
In the 5th Century.
"Down even to the fifth century the observance of the Jewish Sabbath was continued in
the Christian church." Ancient Christianity Exemplified, Lyman Coleman, Ch.26,
sec. 2, p.527.
"In Jerome's day (420 A.D.) the devoutest Christians did ordinary work on Sunday."
Treatise of the Sabbath Day. by Dr. White, Lord Bishop of Ely, p.210.
"For although almost all Churches throught the world celebrate the sacred mysteries
[the Lord's Supper] on the Sabbath of every week, yet the Christians of Alexandria
and at Rome, on account of some ancient tradition, refuse to do this." The footnote
which accompanies the foregoing quotation explains the use of the word "Sabbath" It
says : "That is, upon the Saturday. It should be observed, that Sunday is never called
'the Sabbath' by the ancient Fathers and historians." Sacrates, Ecclesiastical History,
Book 5, chap. 22, p. 289.