Let me ask you a question BW, and this also isn't to start an argument, get anyone angry, etc. But Bobby just brought it up so let's go with it.The apostle John summed it up well. We are not to fellowship those who don't uphold the doctrine of Jesus Christ - and this is not just that Jesus is Christ - per the gospel accounts, the good news and call for repentance to prepare for the Kingdom of God is the doctrine He taught, which goes right along with the sermon on the mount, great commission, promise of God and the warning from Moses when Peter idenfified who 'that Prophet' was.
Most just muse over arguing about what moment a person is 'saved' instead of thinking they have to be hearers and doers of the words that lead to eternal life.
You obviously feel that the command to keep the Sabbath is still in effect, not done away with. You stated that, as John says, we are not to fellowship with anyone who does not uphold the doctrine of Christ. All that being said, do you "have fellowship" with those who don't observe the Sabbath? Do you consider those of us who assemble on Sundays lost?
I have been reading some Messianic Jewish sites, and other Sabbatarian sites, and the consensus seems to be that Sunday worshippers are lost. Just wondering your opinion.