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.)
The civil law, which covers areas such as punishment for theft, adultery, murder, etc., was applicable to a specific nation (Israel) in a given time frame (from the time of Moses to the dispersion of the Jewish nation after the fall of Jerusalem). Granted that legal codes in Christendom drew in part upon Biblical precedents, for example, Justinian's Code and the English common law, the Biblical precedents were not considered binding, but were used as examples. They also used classical Greco-Roman legal codes and Germanic customs, among other sources. If I recall correctly, the position of orthodox Judaism has been that the civil law of the Pentateuch is not binding on either Jew or Gentile until the return of the Messiah. It is a nearly universal position in both orthodox Christianity and Orthodox Judaism that the OT civil law is not applicable to any contemporary society.
The ceremonial law of Israel is what covers such areas as the Temple ritual, the dietary laws, circumcision, etc. Given the Christian position that Jesus Christ's substitutionary atonement is regarded as satisfying God's justice for the believers and will remit the punishment due for their sin, the elaborate rituals of the Old Law were rendered obsolete. Remember that the Judaizers were defeated in the early church at the church council in Jerusalem described in Acts 15. Romans 2:29 notes that circumcision is not of the flesh, but of the heart. Peter's vision in Acts 10 where he heard God stating that there were no unclean animals overrode the Old Testament dietary laws. There are numerous other examples. The position held by the Christian church since Apostolic days has been that the ceremonial law of the Old Testament are not applicable in the current age.
What has not been overturned in the current age and is applicable to all of mankind is the moral law. Most of its precepts are repeated in the New Testament and are affirmed by both Jesus and Paul. The one difference between the Old Covenant and the New Covenant is that we are not saved by the law, but by grace. As Ephesians 2:8-10 states: "For by grace are you saved through faith; and that not of yourselves". The moral law is our guide to God's standards and is binding, but obedience thereto does not save.