If Sabbath keeping is so essential to the Christian life, then why do the apostles not say so? In fact although the word "Sabbath" ("Sabbaton" in greek) is only used extensively in the gospels as it is related to the Jews, it is used only once in the apostles letters from Romans to Revelation, which were primarily to Gentiles. And that is in Colossians 2:16 "Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day."
And along these sames lines Paul commanded: "Who are you to judge someone else's servant? To his own master he stands or falls. And he will stand, for the Lord is able to make him stand. One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind." Romans 14:4,5
According to Paul, the religious observance of the Sabbath is optional and reckoned a gray area. It is in the same kind of category as for example whether it's Ok to drink wine. As such, each should follow his own conscience in the matter and abstain from judging others.
However, there are groups of Christians who claim to be Sabbath keepers and who do judge other Christians based on their ritualistic practices. By judging others in this matter they are disobeying the apostle's commands and as such are found to be hypocrites in that while claiming to keep the letter of the law, they violate the heart of the law by doing what the Bible explicitly states that Christians should not be doing - judging others on an illegitimate basis.
In fact I would even go so far to say that those who hold this kind of legalistic attitude call into question as to whether they have come to truly understand the gospel and the grace of God. The cult of the circumcision tried to influence the Galatian churches introducing the idea that they have to be circumcised and follow the Law of Moses to be saved. Paul wrote the book of Galatians in response objecting to their teachings. One thing he said was:
"But now that you know God-- or rather are known by God-- how is it that you are turning back to those weak and miserable principles? Do you wish to be enslaved by them all over again? You are observing special days and months and seasons and years! I fear for you, that somehow I have wasted my efforts on you." Gal 4:9-11
These people were getting caught up in ritualistic things like observing special days and making those things the basis of their righteousness. It wasn't the practice itself which was the issue here, but the reason why they were doing it. They were not treating these as a gray area, but as essential, which is not Biblical Christianity.
There is one other place where a related word "sabbatismos" is used in the letters. That is in Hebrews 4:9 "There remains, then, a Sabbath-rest for the people of God; for anyone who enters God's rest also rests from his own work, just as God did from his. Let us, therefore, make every effort to enter that rest, so that no one will fall by following their example of disobedience."
However, the Sabbath-rest being spoken of here refers to entering to salvation. It isn't referring to going to church on Saturday or Sunday. The context indicates that entering into his rest means to give up trying to earn salvation by your works, which is consistent with Romans 4:4-8"Now when a man works, his wages are not credited to him as a gift, but as an obligation. However, to the man who does not work but trusts God who justifies the wicked, his faith is credited as righteousness. David says the same thing when he speaks of the blessedness of the man to whom God credits righteousness apart from works: "Blessed are they whose transgressions are forgiven, whose sins are covered. Blessed is the man whose sin the Lord will never count against him."
But those who are trying to obtain righteousness through a legalistic observance of the Sabbath have fallen into the same misconception as the Pharisees. It is just the opposite of what the Sabbath was meant to symbolize. And "Sabbath keepers" often end up doing just the opposite of what they claim. For many try to burden the rest of the Christian community with ritualistic observances and threaten people with eternal condemnation if they don't follow their practices. This is just the opposite of what it means to rest in God's grace.
Furthermore, being judged and accused by others for not "keeping the Sabbath"in accordance with their ritualistic understanding is part of living the Christian life! For Jesus was also accused of such in his ministry. So it should not be surprising if his servants are treated the same.