linssue, I have heard that logic. And as far as salvation goes, it is right: we cannot expect that following the Law will save us no matter how carefully we do it. But this is not a matter of salvation; it's a matter of obedience because we have been saved and declared His children.
If you followed such logic to its conclusion, only two laws might stand at all: to love God with all our beings, and to love our neighbors as ourselves. Would we then find that we could set aside the regulations against murder? Or using the Lord's name to curse another? (That's only one way of taking His name in vain.) Or taking what is not ours?
As it happens, Paul himself warned strongly against such an interpretation. "Do we then make void the law through faith? God forbid: yea, we establish the law." (Romans 3:31) "What shall we say then? Shall we continue in sin, that grace may abound? God forbid...." (ch. 6:1-2) "For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another." (Galatians 5:13)
So what did he mean when he said that the Law was no longer in force? It would seem he meant that the Law's penalties no longer applied--not that the Law itself was done away with, as Jesus agrees in Matthew 5:17: "Think not that I am come to destroy the law, or the prophets..." The clearest passage I can find on this matter is Colossians 2:13-15: "And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses; Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it." In this verse it is not the Law itself that is "nailed to the cross" but rather "the handwriting of ordinances," the indictment against us that declares us guilty to death. Praise God that He paid the price His own justice demands!
So the Law's principles remain, even though their penalty is voided.
Then do we keep them all unaltered, or do we let some of them slip or adjust them to differing conditions? Well, there do seem to be differences; Jesus Himself speaks of "one of these least commandments" (Matthew 5:19) and readily answered when asked which was the greatest. In at least one case, circumcision, Paul says that if Gentiles obey it at all, their salvation is voided. (See Galatians 5:2-12) As for the food laws, Jesus' parable in Mark 7:14-23 and Peter's vision in Acts 10 seem to indicate that they are not, or are no longer, strictly in force.
But the Sabbath is not like that. It is one of the Ten original Commandments, and it was ordained by God Himself as early as Genesis 2:2-3, in the "Days of Creation." (Please; I'm not insisting they were literal days!

) And I find no indication of any change in this commandment in the New Testament, as we have discussed extensively.
So as long as we're keeping one day in seven, I for one will continue to observe it, not according to my own whim, but agreeing with the day that some of the world's best record-keepers, the Jews, agree is the Seventh Day of Biblical times.