Unfortunately Romans 14:1 tells us that the passage deals with MAN'S OPINIONS.
The 4th commandment, is not "MAN'S OPINION" but rather the commandment of God.
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But for the sake of argument, let's take verse 5 and "insert" the Sabbath day in there.
5. One person regards one day above another, another regards every day alike. Each person must be fully convinced in his own mind.
--Right off the bat you should be alerted by the fact that it's saying that "a person regards one day above another" --again you should notice the complete absence of God's regarding. Leviticus 23 clearly tells us that GOD regards the 7th day Sabbath as HIS HOLY DAY. If this isn't enough for ya, let's continue...
Let's take verse 5 and put in the Sabbath day as an example. You would have.
Example 1.
"One person regards the Sabbath day above the rest of the days, and another regards EVERY day as "non-Sabbath days"--working every day, delighting in his own desires every day, carrying heavy burdens every day and never sanctifying any days to the Lord." -- You want to be this guy?
Example 2.
"One person regards the Sabbath day above the rest of the days, and another regards EVERY day as Sabbath days ---Never working, always resting, never causing anyone to work (business transactions), never kindling fires to cook and never spending any time delighting in any of his own desires." -- Perhaps you're tempted to be this guy?
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So you see...putting the Sabbath day in context of Romans 14:5 completely does not work. Verse 1 clearly and thankfully tells us that the entire passage deals with "man's opinions' and not God's commandments---which last time I checked, are not open to "man's discussions".