Mar 7:7 Howbeit in vain do they worship me, teaching [for] doctrines the commandments of men.
This is one of the most unusual translations in the KJV because it ignores the line in Isaiah 29:13 that Christ is quoting. That line is translated in the OT of KJV from the Hebrew as "their fear toward me is taught by the precept of men." If you look at the Greek, you can see how it can be translated the same. The word translated as "worship" primarily means "fear." The word translated as "in vain" also means "falsely" so we easily get "but falsely do they fear me, teaching [my] instructions like human orders." The translation of the full verse of Isaiah, which is quoted in here and in the previous verse (and that we saw before in Matt 15:7-9 in the same Greek) is:
Wherefore the Lord said, Forasmuch as this people draw near [me] with their mouth, and with their lips do honour me, but have removed their heart far from me, and their fear toward me is taught by the precept of men:
The point is that religious leaders tend to teach the fear of God rather than the love of God. They think of God's instruction as human laws, which we must obey out of fear of punishment. They don't see them as instructions given to us out of love and a desire for us to have the best lives possible. God is a father offering advice and direction out of love not a policeman or judge threatening us with the law. This is very much in line with Christ's teaching that the Sabbath was made for men, not men for the Sabbath.
In other words, God.s instructions are not like the orders of a ruler. They are explanations of how the world works, that is, the universal rule. In a sense, they are the laws of nature. We cannot violate them without suffering the consequences any more than we can walk off a cliff without tangling with the law of gravity. God's instructions are about the nature of things not meaningless lines in the sand that he is forbidding us to cross. The commandments exist not to deprive us of pleasures but to enable us to have better lives.
"Do they worship" is from sebomai (sebomai), which means "full of awe, ""feel fear" or "feel shame," before God, "fear to do," and "worship". In Hebrew, the sense of fear is more direct. The single word, yir'ah, which means "fear", "awe," or "fear of God." In the original Hebrew, this line is translated in KJV as saying, that the fear of God comes from the teaching of men, not from God. The Greek could and should be translated this way as well.
"Commandments" is from entalma, which means "to order). The Greek, entalma, only appears here and is not a standard word in Greek. It is translated as a local form of entolê which means "injunction", "order," and "command."
"Of men" is from anthropos , which means "men," people," and "humanity."