>> “As far as I can tell, the Sabbath Commandment is the only one of the Ten Commandments never repeated in the New Testament.” <<
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The Sabbath was universally kept by all who claimed to believe until the Council of Lao DiCea, in 364 AD.
If you love him, you keep his commandments, as he demanded, and as John repeated in his first epistle.
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You are making the assumption that the commandments of Jesus are the same as the commandments of God in the O.T...