At several points, Jesus affirmed the authority of Scripture, which at that time would have been the Old Testament. Citing Psalm 82 in John 10:24-26, Jesus referred to the Psalm as the word of God. Another direct word of the Savior also affirms His view of Scripture. "...until heaven and earth pass away, not the smallest letter or stroke shall pass away from the Law until all is accomplished." (Matthew 5:18) In the previous verse (Matthew 5:17), He stated that He had not come to abolish the Law but to fulfill it. In numerous cases, such as His confrontation with the devil in Matthew 4, his comments to the Sadducees in Mark 12:24, and in the Olivet Discourse in Matthew 24, Christ cited Scripture for moral rebuke, doctrine, and the accuracy of its prophetic writings.
To cite an historical analogy, the ratification of the U.S. Constitution did not make the state constitutions or Anglo-American common law obsolete. While the Federal constitution may have superseded certain portions of the latter writings, they remained valid and lawful, a fact that the authors of The Federalist Papers, who were at the Constitutional Convention and would have known firsthand, pointed out. It would be wrong to say that the U.S. Constitution overruled, say, Pennsylvania's death penalty for murderers, just as the words of Jesus Christ did not overrule the Biblical commandment against adultery. In fact, His affirmation of the whole counsel of Scripture is even stronger than the words of the Constitution, a positive affirmation of its authority.
If what you posit is so, and the Old Testament is Law for Jews and Gentiles alike, then why do so many Christian sects "pick and choose" which part of the bible to follow and which not?
It seems to me that many of the laws of the Old Testament are not being followed.
For instance: Say to the people of Israel, If any one sins unwittingly in any of the things which the LORD has commanded not to be done, and does any one of them, if it is the anointed priest who sins, thus bringing guilt on the people, then let him offer for the sin which he has committed a young bull without blemish to the LORD for a sin offering.
Where are the bulls? It's been a long time since I've seen a burnt offering. There should be huge fires out in Colorado!
Or: "If a woman conceives, and bears a male child, then she shall be unclean seven days; as at the time of her menstruation, she shall be unclean. Then she shall continue for thirty-three days in the blood of her purifying; she shall not touch any hallowed thing, nor come into the sanctuary, until the days of her purifying are completed. But if she bears a female child, then she shall be unclean two weeks, as in her menstruation; and she shall continue in the blood of her purifying for sixty-six days."
Are we making sure we keep all menstruating women and new mothers* out of the churches? After all, they are unclean.
The Bible tells us so.
(* This is followed, to some extent, in the Orthodox church. New mothers have to have a cleansing ritual performed before they can enter church after giving birth. But no one checks to see if I'm menstruating.)