The last explicit mention of the Sabbath in the NT is in Hebrews (p. 280)—its theme, there remains a Sabbath rest for God’s people. (4.9). Tonstad suggests that the Sabbath which remains refers to a sabbath which can now be enjoyed by Christians and celebrated by Christians, even though the ‘rest’ that the author has in mind has yet to come to fruition and in fact awaits the return of Christ. And we hear not one word about the fact that the audience for this sermon is probably Jewish Christians tempted to go in a retrograde motion back into Judaism, nor even a peep about the obsolescence language when it comes to the Mosaic covenant and its inauguration of Sabbath observance. For Tonstad, Sabbath proclaims the faithfulness of God, his reliability.
Having taken up the challenge of showing that the Bible favors sabbitarianism for one and all, Tonstad then still has a huge task before him to which he devotes several chapters (pp. 298ff.)— how then did it happen that the church abandoned the Sabbath for the Lord’s Day or replaced the former with the latter, or subsumed the former into the latter? How could that happen if God expected of all creatures great and small a perpetual Sabbath praxis?
Despite sporadic evidence of continued Sabbath observance by Christians into the 4th century, perhaps principally by folk like the Ebionites, and in places like Syria, Ethiopia, and Egypt, Tonstad admits that from the middle of the 2nd century A.D. the trend is lopsidedly in favor of Sunday rather than Sabbath observance. (p. 301). Noting that the 2nd century theologians don’t debate whether Sabbath should be kept or not, the praxis just slips into oblivion and no one can say exactly when Sunday worship began. (there are hints already in the NT). Justin Martyr is however quite clear— we observe Sunday because not only is it the day God began to create the world, it is the day Jesus rose from the dead (First Apology 67), but even clearer and earlier is the pronouncement of Ignatius of Antioch to the Magnesians “those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord’s Day, on which our life has sprung up again by Him and by His death” (Mag. 9.1). Tonstad will put this down to waning Jewish influence in and on the church, alongside of rising anti-Semitism as well. The parting of the ways led to the parting of the days. Tonstad even suggests Sunday observance was something Christians borrowed from Mithraism or from the cult of Sol Invictus pp. 308-09, 314. There is a straight repudiation, without evidence, of O. Cullmann’s assertion that Sunday is a specifically Christian festival day (p. 309). And he follows J. Moltmann’s suggestion that widespread Sunday observance came by royal decree of Constantine on March 3 312 when he urges all judges, townspeople and all occupations to rest on the honorable day of the sun. This can be called Adventist polemics since there is already evidence not only in Ignatius and Justin, but in Pliny (on the first day of a week, singing songs to Christ as to a God) that long before Constantine in the province of Asia, Bythinia, Syria and elsewhere Christians were worshipping on Sunday.
The likely reality is that when a large number of Gentiles became Christian through the the Pauline and other missions, Gentiles who had had no synagogue connections previously, they came to Christian worship and did Christian worship outside the context of Sabbath observance, and this was not critiqued by Paul or his co-workers, only by the Judaizers from Jerusalem. There is no evidence at all that mixed Jew and Gentile congregations in Ephesus or Corinth or Philippi continued to observe the Sabbath as part of their Christian praxis, though undoubtedly various Jewish Christians and God-fearers probably continued to go to the synagogue.
Tonstad, in Chapter 17 (pp. 316ff) suggests that anti-Judaism must be put down as a main reason for the eclipse of Sabbath praxis amongst Christians, but another is Hellenism, more particularly Platonism with its devaluation of the material world, the human body which is seen as a prison house of the immortal soul, and so on. The early church imbibes and adopts a Platonic world view without seeing its incapability with a Biblical doctrine of the goodness of creation, of matter, of bodies, of resurrection. The revised Platonic Gospel promises escape from creation into a disembodied heaven not the transformation of creation. On these terms, it is hard to explain why highly Hellenized Diaspora Jews would continue to observe a Sabbath, continue to affirm the body, bodily life, and even a doctrine of resurrection as a litmus test of sorts after A.D. 70. The answer is that a theology of Sabbath is not the same as a theology of creation, though the former is linked to, and to some extent grounded in the latter. One can have a perfectly robust theology of Sabbath keeping, and a deficient theology of creation, or a robust creation theology, with no theology of Sabbath at all, as Genesis as a whole does not affirm much less mandate Sabbath keeping, and Gen. 2.1-3 doesn’t say God instructed his human creatures to do so.
A theology that sees no immortal virtue in the body is a theology that can justify its neglect or abuse. And Tonstad suggests that Platonic medieval Christianity aided and abetted things like the Black Plague because medicinal research was not seen as essential to life or very important and studying the human body itself was seen as a sacrilege. (pp. 332-338). “The medieval idea of the body and the earth had a paralyzing impact because it did not offer any incentive to improve people’s lot in this life” (p. 336).
P. 342ff. presents us with a test case of Tertullian who is seen as a rare exception—a theologian who was earth and body affirming, though he never lifted a finger on behalf of the Sabbath. He was anti-Greek philosophy and its body soul dualism. He even affirmed a material soul.