Isaiah 56 is one of many descriptions of the kingdom age. The "new covenant" was given to Judah and to Israel; not to grace age Christians. This passage is dealing with the earthly Gentiles of the millennial age and not with the church age saints. At that time all the nations of the earth will also come to Jerusalem on the feast of tabernacles to celebrate that feast with Israel. If we are obligated to the Sabbath because of Isaiah 56 (which Paul says we are not - Colossians 2:16-17), then we are also obligated to go to Jerusalem every fall in order to keep the feast of tabernacles. But there is much more we would also be required to do if you go by these kingdom passages. We cannot take part of it without taking all of it lest we practice dissimulation.
In effect, this would put us entirely under the law again, something we have been delivered from (Romans 6:14-15; Galatians 3:23-25). And when we come back under the law, we have put ourselves back under the curse (Galatians 3:13). The whole idea of us today being under Sabbath Worship is an affront to the blood of Christ and His sacrifice on the cross. It says that His grace was not sufficient.
