This is a very good question. Let’s begin making our way toward an answer by suggesting a basic rule of thumb for properly applying the Old Testament today, which is this: We should assume that whatever God once required of his people is still required unless he has altered the requirement in some way. In other words, our operating assumption ought to be one of continuity between the Old and New Testaments. Everything God required of his people in the Old Testament should be assumed to have an ongoing obligation for us today unless the New Testament teaches us otherwise. This is implied in Jesus’ Sermon on the Mount when he says:
Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven (Matt. 5:17-19).Here we find the assumption of continuity with a hint that portions of the Law and the Prophets may be “accomplished” in such a way as to make them no longer necessary for God’s people to observe. What portions might these be?
When we read the Old Testament law carefully it becomes apparent that not all the commandments are of the same kind. Some of the commandments clearly have a moral purpose, which is to say, the behaviors they require or forbid are inherently moral or immoral. We say that these commandments are a part of the “moral law.” What commandments might these be? Briefly, they are laws that require us, first, to do our duty to God, second, to do our duty to men. Let me give a few examples.
Commandments that have to do with our duty to God are those that require us to love, trust, and obey him, and give him our ultimate and exclusive allegiance. “You shall have no other gods before me” (Ex. 20:3). “You shall love the LORD your God with all your heart and with all your soul and with all your might” (Deut. 6:5), and so on.
Commandments that have to do with our duty to men are those that forbid us to harm our neighbor. “You shall not murder” (Ex. 20:13). “You shall not oppress your neighbor or rob him” (Lev. 19:13). Also, commandments that require us to help our neighbor in his time of need whenever it is in our power to do so. “If…one of your brothers should become poor…you shall open your hand to him and lend him sufficient for his need” (Deut. 15:7, 8). Here’s another, “You shall not see your brother’s ox or his sheep going astray and ignore them. You shall take them back to your brother” (Deut. 22:1).
These commandments that speak of our duty to God and men are rooted in our nature and relations to one another as moral beings; and as long as our nature and relations to one another remain what they are—which is forever—they can never be done away with.
But there is another category of law in the Bible besides the moral law. We call it the “ceremonial law,” and it is no longer required of God’s people. The ceremonial law has to do with the various rituals and observances associated with the temple and the priesthood and with Israel’s separation from the nations. These include such things as:
(1) Sacrifice and offering—we no longer have to bring animal sacrifices to a temple
(2) Purification rites—the various sprinklings with blood and washings with water
(3) Dietary restrictions—in Mark’s Gospel we read of how Jesus declared all foods clean (Mk. 7:19)
(4) Laws associated specifically with Israel’s possession of the land (laws governing the sale of land and land returning to the original owner in the year of jubilee, etc.)
In sum, all of these laws that do not have a clear moral dimension but have to do with rituals associated with the temple and with marking Israel off as a distinct people are no longer binding. But those laws that command our obedience to God and our just and equitable treatment of our neighbor are still in force.
If you simply start with the Ten Commandments and their logical implications, you’ll do all right.