It's a popular "trick" for ex-vangelicals and atheists to discredit New Testament commands by comparing them to Old Testament laws.
For example, the late Rachel Held Evans claimed to be testing the validity of Scripture by reenacting certain commands for women. But she didn't differentiate between the old and new covenants. No wonder she concluded "Despite what some may claim, the Bible’s not the best place to look for traditional family values as we understand them today." To read a biblical response to her irrelevant "experiments" see Muddled Views of God, Women and Scripture.
It's not hard to understand the differences between Old and New Testament laws if we approach them without an agenda.
✔ Old Testament dietary, cleansing, and ceremonial laws served an important purpose: to prove that external law-keeping can't change hearts or make us right with God (Romans 3:19-20; Romans 4:13-14; Galatians 3:23-25). Christ didn't abolish these laws, but He fulfilled them (Matthew 5:17).
✔ Civil laws were given to govern the nation of Israel for a specific time in history: In order to set the nation of Israel apart from pagan cultures, God gave them strong laws. For example, the Old Testament nation of Israel held the death penalty for adultery (Leviticus 20:10). If you doubt the temporary nature of this law, read the story of Jesus and the woman caught in adultery (John 8:1-11). The principle is universal: adultery is a sin. But the punishment in Leviticus is not permanent.
✔ External dietary and cleansing laws: These were a "shadow of things to come," showing us that mankind is "unclean." Christ fulfilled these laws for us because He is the only One who can make us clean enough to stand in God's presence. Compare Leviticus 22:4-8 with Hebrews 10:1-25. The end of dietary laws is explained Mark 7:18-19, Matthew 15:11 and Acts 10:9-16.
Mark 7:18-19: “Are you so dull?” [Jesus] asked. “Don’t you see that nothing that enters a person from the outside can defile them? 19 For it doesn’t go into their heart but into their stomach, and then out of the body.” (In saying this, Jesus declared all foods clean.)
The words in parenthesis are in the Bible, not added. It is found in the original Greek.
1. External laws were fulfilled in Christ.
Galatians 2:21-23: “Do not handle! Do not taste! Do not touch!”? These rules, which have to do with things that are all destined to perish with use, are based on merely human commands and teachings. Such regulations indeed have an appearance of wisdom, with their self-imposed worship, their false humility and their harsh treatment of the body, but they lack any value in restraining sensual indulgence.
Matthew 15:11: What goes into someone's mouth does not defile them, but what comes out of their mouth, that is what defiles them."
See full passage here: Matthew 15:17-20.
For more details, see Bite Size Bible Study: Old Testament Laws Series.
20 To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. 21 To those not having the law I became like one not having the law (though I am not free from God’s law but am under Christ’s law), so as to win those not having the law.
If you would like to do a more thorough study of how the law impacts Christians, I encourage you to study Romans 7, Romans 10:4, Ephesians 2:14-16, and Galatians 2:16.
copyright 2013, Gail Burton Purath, BiteSizeBibleStudy.com, updated in 2020