When Christians engage in online discussion of the Bible we are often accused of choosing Bible verses that support our postion and ignoring those that oppose it. Usually this takes the form of quoting some law found in the Old Testament and asking why we don't practice it. Since we consider all of the Bible, including the Old Testament, to be the Word of God this is a reasonable response and I will try to answer it.
God has given two kind of commands in the Bible. One kind consists of moral laws that are given to all people and are always in effect. Jesus said that these laws can be summarized in two commands:
"You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love your neighbor as yourself."
All of the other moral laws of the Bible are simply detailed instructions on how to carry out these commands.
In addition to giving universal commands that apply to everyone God also gives commands to specific individuals and groups that apply to them but to no one else. For example, he told Noah to build an ark to keep all life from being destroyed in the flood. This command was for Noah and his family alone and God never intended for anyone else to obey it.
After he had delivered the Israelites form slavery in Egypt he chose them to be his people and gave them laws which they were required to obey. These laws included all the moral laws which apply to everyone but they also include commands which weren't ever given to anyone else. The question is, which of these commands must we obey today and which were only for Israel and don't apply to us?
Israel was a nation but the Church isn't a nation but is a body of believers who are subject to the laws of the nations in which they live. A nation has the authority to enforce its laws and punish those who violate them, even executing them if their offenses are serious enough. A church doesn't have the authority to impose any kind of physical punishment but is limited to expelling from its membership those who continue in sin and refuse to repent. A nation has the right to engage in military activities to protect itself against other nations. Christians are engaged in warfare but it is spiritual, not physical. If church leaders in the past had kept this distinction in mind the Crusades and the Inquistion probably wouldn't have taken place.
God commanded the Israelites to offer animal sacrifices to atone for their sins and he established a priesthood to carry out these sacrifices. These sacrifices were intended to show what Christ would do when he died for our sins and now that he has done this they are no longer needed. The book of Hebrews explains how he fulfilled the sacrificial laws.
The fact that we are not required to follow some of the laws in the Bible doesn't mean that we shouldn't study them. Second Timothy 2:16,17 says,
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work.
Many of the commands were intended to illustrate spiritual truths. For example, Deuteronomy 25:4 forbids muzzling an ox which is treading out grain. In 1 Corinthians 9:9 and 1 Timothy 5:8 Paul quoted this command to show that Christians are to financially support those who work full time at preaching the gospel. When you read a command in the law you should think not just about its literal meaning but what spiritual truth it illustrates.
Here are some some other examples of commands which we no longer have to follow literally but which teach principles which we should follow.
"When you build a new house, you shall make a parapet for your roof, that you may not bring the guilt of blood upon your house, if anyone should fall from it." Deuteronomy 22:8.
When this was written houses were built with flat roofs and the roof was used as another room in the house so obviously a railing was needed to protect the people on it. Since we don't build houses like this today we don't have to follow this law literally but we can learn something from it that does apply today. If God had wanted to do so he could have used his power to protect the people from falling off their roofs and a railing wouldn't have been necessary, but he has chosen not to do this. We learn from this that he expects us to foresee possible dangers and take precautions to guard against them. We should not use the fact that he has promised to protect us as an excuse for carelessness.
"You shall not sow your vineyard with two kinds of seed." Deuteronomy 22:9.
When Jesus explained the parable of the sower he said that the seed was the Word of God. When we are telling the gospel or teaching the word we must be careful to teach only what the Bible says and not allow any human views to become mixed in with our teaching.
"You shall not plow with an ox and a donkey together." Deuteronomy 22:10.
An ox was a clean animal which could be eaten or offered as a sacrifice. A donkey was unclean and couldn't be sacrificed to God. Paul might have had this verse in mind when he wrote 2 Corinthians 6:14, "Do not be unequally yoked with unbelievers." The work of God is to be carried out only by Christians and we must avoid anything which we bind us too closely with unsaved people. This verse has often been used to show that a Christian shouldn't marry a nonChristian, and while this is true it applies to other situations as well. One area where Christians have violated this principle in the past is in making Christianity the official religion of a nation. The result has always been that the unsaved citizens of the country have been able to use this union to lead the church away from following the Bible. Christians should use their influence to ensure that the laws of their country are in accord with God's standards of right and wrong but they should never seek or accept government in the primary work of the church, which is preaching the gospel and teaching the Bible.
"You shall not wear cloth of wool and linen mixed together." Deuteronomy 22:11.
Revelation 19:8 says that find linen is the righteous deeds of the saints. Ezekie 44:17,18 says that when priests enter the sanctuary that must wear only linen and not wool, because wool will cause them to sweat. This seems to show that this law is a warning not to try to approach God on the basis of what we deserve because of our works, but to rely entirely on the righteousness which comes from faith in Christ.
This is just a small sample of the things that we can learn from a study of the commands that God has given.