Why did the Council of Laodicea condemn "Judaizing" Sabbath rest?

In Christianity, it has been central tenet that Christians are free from the Law since the very beginning. The New Testament very much makes it clear that observance of holy days, including the Sabbath, is optional for Christians. Paul makes it clear in Romans 14.

One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind. Whoever regards one day as special does so to the Lord. Whoever eats meat does so to the Lord, for they give thanks to God; and whoever abstains does so to the Lord and gives thanks to God.

The vast majority of Christians have not celebrated the Jewish Sabbath since the time of Paul, based on this teaching. Over time there has arisen a tradition of observing a day of rest on the first day, and this has grown up and been strong in the Catholic church, but also in many others (Scottish Presbyterians are very fervent in keeping, so please don't try to tell them it's a Catholic tradition).

The pronouncement at the council of Laodicia is not against those who keep a day of rest. It is against those who insist that keeping a Sabbath is necessary for the Christian. Such an insistance is viewed as a return to the jewish Law, and thus entirely incompatible with Christianity (other have of course disagreed with them).

That the council had to legislate on the matter shows that some Christians were, indeed, resting on the sabbath.

Socrates Scholasticus, writing in the 5th century, states that churches in many places honored the sabbath with religious assemblies:

For although almost all churches throughout the world celebrate the sacred mysteries 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. The Egyptians in the neighborhood of Alexandria, and the inhabitants of Thebais, hold their religious assemblies on the sabbath, but do not participate of the mysteries in the manner usual among Christians in general... (Church History 5.22)

So attitudes toward the sabbath in early Christianity were diverse.

The bishops of Laodicea probably had multiple motives for legislating as they did. Jew-hatred cannot be ruled out as one factor.

"[W]e too would observe the fleshly circumcision, and the Sabbaths, and in short all the feasts, if we did not know for what reason they were enjoined [on] you—namely, on account of your transgressions and the hardness of your heart. . . . [H]ow is it, Trypho, that we would not observe those rites which do not harm us—I speak of fleshly circumcision and Sabbaths and feasts? . . . God enjoined you to keep the Sabbath, and imposed on you other precepts for a sign, as I have already said, on account of your unrighteousness and that of your fathers . . ." (Dialogue with Trypho the Jew 18, 21 [A.D. 155]).

It would seem to me that the argument given by Justin Martyr is that because the law (Old Testament and specifically Genesis-Deuteronomy) had been fulfilled by Christ, then it was no longer necessary to keep them, seeing as they were fulfilled in Christ. Now that Christ has become the lamb of God, we no longer need to keep passover...etc. It appears that this was his logic behind the 7th day sabbath, saying Christ is our sabbath and when we come to him we enter into that rest. It becomes apparent then, that to impose these things (circumcision, sabbath, passover, etc.) would be taking us back and placing us under the requirements of the law and tying our doing of those deeds to our justification/salvation.

The sabbath thus being done away with, having been fulfilled, the Christian must find a day to worship on.

"But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Savior on the same day rose from the dead" (First Apology 67 [A.D. 155]).

This day becomes Sunday, for various reasons. It is the beginning of the Creation of God, and God was birthing new creations in Christ (2 Corinthians 5:17 ESV). This is confirmed by Athanasius (who was a contemporary of the council and probably took part.) saying:

"The Sabbath was the end of the first creation, the Lord’s day was the beginning of the second, in which he renewed and restored the old in the same way as he prescribed that they should formerly observe the Sabbath as a memorial of the end of the first things, so we honor the Lord’s day as being the memorial of the new creation" (On Sabbath and Circumcision 3 [A.D. 345]).

And it was the Day of the Resurrection, the same being shown to be the beginning of new life after we have died to the flesh. (Romans 6:4-11)

Hope this helps.

Judaizer is a term for those early Christians who held that adherence to Mosaic law was necessary for salvation. Observance of these laws (such as circumcision and the distinction of clean and unclean foods) was proscribed in Acts 15 and 11, respectively.

The New Testament distinguishes the Lord's Day from the Sabbath, and it commemorates the day of the Resurrection. While the topic has only a few scriptural references (e.g., Acts 20:7, Rev 1:10), the testimony of the early Church Fathers is consistent.

The intent by the Council in using the term Judaizer seems clear: Christianity is not just another sect of Judaism. Jews worship on the seventh day, the Sabbath. Christians worship on the eighth day of Creation, the Lord's Day, to commemorate the day of the Resurrection. Those who teach otherwise must be called out.

You can find the complete canons of the Synod of Laodicea here.