We aren't under the law anymore, we are under grace. If we were still under the laws in the old testament, then women would be unclean during their periods and after childbirth. You couldn't eat shellfish or pork or anything else deemed in the old testament as unclean. There would still be stonings today.
I don't follow OT law because the recent NT writings specifically tell me not to. For example:
“For sin shall not be master over you, for you are not under law, but under grace.” - Rom 6:14-15
“But now we have been released from the Law, having died to that by which we were bound, so that we serve in newness of the Spirit and not in oldness of the letter.” - Rom 7:6
"knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified.” - Gal 2:16
“But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed. Therefore the Law has become our tutor (to lead us) to Christ, that we may be justified by faith. Now that faith has come, we are no longer under a tutor. " - Gal. 3:23-25
Over in Hebrews 8 (NT), Paul quotes a long section of Jeremiah (which is in the OT). That section is as follows:
“The days are coming,” declares the Lord, “when I will make a new covenant with the people of Israel and with the people of Judah. It will not be like the covenant I made with their ancestors when I took them by the hand to lead them out of Egypt, because they broke my covenant, though I was a husband to them,” declares the Lord. “This is the covenant I will make with the people of Israel after that time,” declares the Lord. “I will put my law in their minds and write it on their hearts. I will be their God, and they will be my people. No longer will they teach their neighbor, or say to one another, ‘Know the Lord,’ because they will all know me, from the least of them to the greatest,” declares the Lord. “For I will forgive their wickedness and will remember their sins no more.”
So even if a Christian tried to follow the OT, the OT itself commands them to follow the "new covenant" instead. So if you believe (and I think Paul makes pretty clear) that a new covenant has been established with the arrival of Christ, you can't really follow the "OT law" because that very same law says it's okey-dokey to ignore it.
Paul then adds this comment:
By calling this covenant “new,” he has made the first one obsolete; and what is obsolete and outdated will soon disappear.
Actually the entire Book of Hebrews is practically an essay on why the law no longer applies. Paul goes in and very meticulously pulls apart all sorts of different elements of the law (like high priests, and tabernacles) and explains how they are totally replaced by various elements of Christianity.
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