Anyone who has studied the Christian heresies in the early centuries of the church is aware that the many errors that arose in the church actually helped the church to define who Jesus is. In the long run, the church was actually helped by being forced to better interpret the Bible.
A recent error (I will not call it a heresy) is called Christian Reconstruction or Theonomy. It is the notion that the OT civil laws should be adopted wholesale by all nations today. They say that only the sacrifices and feasts have been fulfilled by Jesus and are the only part of the OT Law that has been abrogated. Hence, the civil aspect of the Law is still in force and God expects us to obey it.
Most Christians disagree with this, saying that only the moral aspect of the Law is still valid today. They say that the laws that God gave to Israel were only for them though we can learn principles of wise and just government from them. I agree with this latter approach.
It is a small minority, almost a fringe element, that holds that to Theonomy but many others use Theonomic principles when it suits them. Let's take, for instance, the issue of immigration in America. We have laws restricting immigration though there is controversy both about the laws themselves and the enforcement of those laws.
Many want firmer measures to enforce those laws; others have tried to use to the OT laws to support the opposite position. They want an open door policy on immigration. The OT does say to welcome aliens and strangers and take good care of them. There was definitely an 'open immigration' policy in Israel.
So, they say, the US should adopt the same policy and let multitudes to remain in this country who have not followed legal immigration procedures. You can make this argument based in OT law, but are you willing to be consistent? Will you apply all OT laws and punishments? Are you suggesting that Theonomy is the solution to immigration but not to anything else? Do you want to stone adulterers and homosexuals? I doubt it. When Uganda recently passed harsh laws forbidding homosexual activity, Christians said that this was wrong. Some of these same Christians are supporting open borders based on OT law.
When we say 'let's examine what the Bible says about X,Y or Z' should we not have a clear understanding about why certain OT laws existed? (And let's not condescendingly attribute a lack of mental and moral development to ancient peoples.) One of the main purposes that Israel existed was to bring the revelation of the true God to the world. They did not go out and evangelize. Rather they were to welcome all to come and learn about Him.
The call of the church is to make disciples of all nations, going out into the world to preach the gospel. It is the church that has to have an open door policy, not the nation in which it resides. The most proper application of the OT law regarding immigration is towards the church and not secular governments. The purpose of the law is to propagate the knowledge of God. That is now done by the church, not the nation of Israel.
Now some will say that the NT always leans on the side of compassion rather than justice and that our laws should be compassionate. But is that the purpose of law, any law? Laws are to establish justice, not compassion. Mercy ought to be shown sometimes in the administration of the law, but the laws themselves exist to give order to a society and to punish wrongdoing. When a person enters the country illegally then he has done wrong. Expulsion is not too harsh a punishment.
Governments administer justice; the church administers mercy. We need to remember this "separation" of church and state functions. There is no contradiction between the church feeding and clothing an illegal immigrant and the government arresting and deporting the same person. Both ought to be done. It is no different from prison ministries. We go to prisons to minister to those who are suffering, though some of that suffering is caused by the government enforcing the law in their case. We do not suggest that they be let out of prison because the Bible teaches 'compassion'.
So if we want to make a case for an open immigration policy we must make it on other grounds. We must be careful not to use the OT law in a inappropriate manner. Paul warned against those who would be "teachers of the Law, even though they do not understand either what they are saying or the matters about which they make confident assertions. But we know that the Law is good, if one uses it lawfully." (1 Timothy 1:7-8) There is a proper use of the Law. Let us use it wisely and not for merely promoting our own politics.