It's helpful to make a distinction between three things that we mean when we talk about the laws, rules and regulations in the Bible. They are (1) the legal system of ancient Israel, (2) the ceremonial law of the Old Covenant and (3) the moral law.
Legal: The legal system of ancient Israel are the laws about how people should live as God's people in the land that God gave to the Israelites. Things like how adultery, theft, blasphemy and murder should be punished.
Religious: The ceremonial law of the Old Covenant specified what had to be done in order to relate to God – which sacrifices should be made, by whom and when etc. Things like what the High Priest would do on the day of atonement, or circumcision, or the kind of offering you make to say thank you to God after the birth of a child.
Moral: The moral law is about what is right and what is wrong. How do good people behave? What behaviour is acceptable in God's eyes? How should I treat my neighbour?
The legal system of ancient Israel became redundant when the nation was taken into captivity. You have to have authority to govern in order to administer a legal system – and they didn't have that. Hence even Jesus was not bound by it: he told people to pay taxes to Caesar (which Jewish Law didn't demand, but Roman law did – Matthew 22:17-22) and refused to stone a woman caught in adultery (John 8:2-11) even though the law demanded it (Lev 20:10).
The ceremonial law of the Old Covenant became redundant because Jesus made a new covenant: Hebrews 10:1 points out “The Jewish Law is not a full and faithful model of the real things; it is only a faint outline of the good things to come.” and in Hebrews 8:13 he says “By speaking of a new covenant, God has made the first one old; and anything that becomes old and worn out will soon disappear.” The author of Hebrews spends most of the time from chapter 7 through to chapter 10 explaining how the Old Covenant was like a human model to help us understand what Christ did for real.
The moral law is the stuff that doesn't change – it includes stuff that Moses is told which is re-affirmed by Jesus (e.g. Matthew 5-7, Sermon on the Mount) and Paul (e.g. Ephesians 5-6).
It's not always easy putting stuff into neat boxes – adultery, for example, is a definite no-no for all time (ie. Moral law) but the death penalty for adultery is part of the legal system of ancient Israel – not ours (ie. Legal). Hence it would be totally wrong for us to stone adulterers. Laws about purification after sex are part of the ceremonial law (ie. Religious) and hence do not apply either: we are made clean by the blood of Christ. Simulating it with water is useless and would just encourage superstition.
When trying to work out which category a particular instruction falls into, it's helpful to look at where it is in scripture:
- If it's in the letters in the New Testament, it almost certainly applies to us (unless, for example, it's Paul asking Timothy to bring him his cloak & parchments – 2 Tim 4:13).
- If it's Jesus speaking then it's highly likely – although there are times when he's talking to the Pharisees about how to keep the Old Covenant which fall under the Ceremonial Law.
- If it's in the Old Testament, then what kind of instruction is it? Is it part of the legal system, or is it a religious regulation that Jesus has replaced, or is it about the way the world should be, ie. moral?
It's also worth asking – even when the instruction is doesn't apply directly to you – whether there's some transferable principle that does.