The Entire Old Testament Is for Christians Today
The Old Testament chronicles God's dealing with the patriarchs and with his chosen nation Israel. So much so that entire law codes for Israel appear in Exodus, Leviticus, Numbers, and Deuteronomy. Here's an example of instruction in the book of Deuteronomy: "When you build a new house, make a parapet around your roof so that you may not bring the guilt of bloodshed on your house if someone falls from the roof" (Deut 22:8). Based on this text, here's a question. Where's your parapet? Or does the Old Testament's historical context mean that it does not apply directly to you? Far from it! On the contrary, the whole Old Testament wholly and completely applies to you and me.
But Let's Make It More Difficult
If you cannot apply the Old Testament to your life because it was written to people who lived in the deep past, then neither can you apply the New Testament to your life for the same reason. When Paul wrote to the Corinthians, he wrote about specific issues local to the church in Corinth. For example, 2 Corinthians largely relies on a letter that Paul sent to the church there, a letter that we have no knowledge of: "For I wrote to you out of much affliction and anguish of heart and with many tears, not to cause you pain but to let you know the abundant love that I have for you" (2 Cor 2:4). The New Testament is not any easier to apply directly to your life than the Old Testament, if your reasoning is that the Old Testament was written to ancient peoples in a specific historical context.
So None of It Applies to Us?
Not at all! All of it applies to you wholly and completely. But the question is how. Christ established a new covenant, a new agreement with humanity by his death and resurrection. The old way, the old covenant is no longer in effect. So, the Old Testament law code no longer binds us to obedience as part of the nation of Israel and with legal repercussions for disobedience. But while the teeth of the law have had their edge dulled, they law still applies. Here's how: the law (and really the whole Old Testament) applies us as wisdom and as prophecy.*
The Law (& Old Testament) as Prophecy
The Old Testament often speaks of what has happened, is happening, and will happen. It's prophetic. And its prophetic force, its forthtelling character remains the same today. The Old Testament quite simply speaks of God and reveals God. And sometimes this means revealing what God will do in the future. Jesus takes this fulsome understanding of the Old Testament in Matthew. 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" (Matt 5:17–18). And so it is that he fulfilled the law and prophets by doing the law, by repeating it, and by showing himself to be the object of prophecy. When Matthew uses the word "fulfill" in reference to Jesus, he often uses the word in reference to Jesus' actions of doing and accomplishing the Old Testament. For example, when Jesus left Egypt, Matthew writes, "This was to fulfill what the Lord had spoken by the prophet, Out of Egypt I called my son'" (Matt 2:15). When Isaiah speaks of the blindness of the people Isaiah 6, that's about Jesus speaking parables according to Matthew (See Matt 13:14). Jesus' speaking in parables also fulfills the words of Asaph in Psalm 78** according to Matthew 13:34–35, "All these things Jesus said to the crowds in parables; indeed, he said nothing to them without a parable. This was to fulfill what was spoken by the prophet: 'I will open my mouth in parables; I will utter what has been hidden since the foundation of the world.'" Jesus does what Israel did in the Old Testament but in a perfect, fulfilling way. What Jesus does, says, and is is fulfilling the Old Testament. He sums up the Old Testament. It's about him. So he says, "You study the Scriptures diligently because you think that in them you have eternal life. These are the very Scriptures that testify about me" (John 5:39). And so does Moses, as Jesus underscores: "If you believed Moses, you would believe me, for he wrote about me" (John 5:46). The Old Testament is about Jesus. This prophetic application of the Old Testament came easily to the first generation of Christians. Who brought Israel out of Egypt, led them in a cloud, and destroyed the unfaithful among Israel? Well, according to Jude, it's Jesus: "Now I want to remind you, although you once fully knew it, that Jesus, who saved a people out of the land of Egypt, afterward destroyed those who did not believe" (v. 5). During their wilderness wanderings, who provided for the Israelites? According to Paul, it's Jesus, "For they drank from the spiritual Rock that followed them, and the Rock was Christ" (1 Cor 10:4). To whom did Israel put to the test in the wilderness? According to Paul, it was Jesus, "We must not put Christ to the test, as some of them did and were destroyed by serpents" (1 Cor 10:9). Jesus. Jesus. Jesus. That's the what the Old Testament was about. It's prophetic of him. He fulfills it by his actions, by his words, and by his person. All that he does and is is what the Old Testament speaks of. It's comprehensive. It's all about him. So the Old Testament speaks directly of Christ, and knowing God and Jesus Christ is eternal life (John 17:3). So, it's for you directly in this way.
The Law (& Old Testament) as Wisdom
Secondly, the Old Testament as wisdom directly applies to believers. As Paul says, "Now we know that the law is good, if one uses it lawfully" (1 Tim 1:8). So use the law lawfully. How? By using it a divine guide to virtue. Paul clarifies, "the law is not laid down for the just but for the lawless and disobedient" (1 Tim 1:9). Paul also has a longer reflection of on the law (i.e., the books of Moses that comprise Genesis through Deuteronomy) in 1 Corinthians 10. Let's step back and read it together:
For I do not want you to be unaware, brothers, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same spiritual food, and all drank the same spiritual drink. For they drank from the spiritual Rock that followed them, and the Rock was Christ. Nevertheless, with most of them God was not pleased, for they were overthrown in the wilderness.
Now these things took place as examples for us, that we might not desire evil as they did. Do not be idolaters as some of them were; as it is written, “The people sat down to eat and drink and rose up to play.” We must not indulge in sexual immorality as some of them did, and twenty-three thousand fell in a single day. We must not put Christ to the test, as some of them did and were destroyed by serpents, nor grumble, as some of them did and were destroyed by the Destroyer. Now these things happened to them as an example, but they were written down for our instruction, on whom the end of the ages has come. Therefore let anyone who thinks that he stands take heed lest he fall. No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it. (1 Cor 10:1–13)
Mark these two phrases:
1. Now these things took place as examples for us, that we might not desire evil as they did.
2. Now these things happened to them as an example, but they were written down for our instruction, on whom the end of the ages has come.
These things written in the law are examples for us. But not just examples. Paul says, "They were written down for our instruction, on whom the end of the ages has come." The proposition here is so important: "for." The Old Testament law (in this case, Numbers) was written for us. Who? Us, upon whom the end of the ages has come. Stop. That's us. Christians. We participate in the final age (see Rom 6). We are new creations; the old age has passed away (2 Cor 5; cf. 2 Cor 4). The Old Testament, including the law, was written for us to teach us to avoid evil.
Ultimately, The Old Testament Makes Us Wise unto Salvation
Perhaps the best way to sum up the purpose of the Old Testament and how it applies to us is this: it makes us wise for salvation. Consider Paul's words in 2 Timothy 3:14–15, "But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus." At this point, the New Testament was not written. So Paul refers to the Old Testament. And he claims that It makes "wise for salvation through faith in Christ Jesus." How? By prophetically speaking about Christ and by giving you the wisdom needed to live in a Christlike manner. That's how the Old Testament applies directly to you wholly and completely.
But the Parapet!
So how does Deuteronomy 22 apply to us? How does "When you build a new house, make a parapet around your roof so that you may not bring the guilt of bloodshed on your house if someone falls from the roof" work out in our lives? Don't build a structure that is not safe nor let someone stand on it. Do the work necessary for the good of your neighbor. And in so doing, you will love your neighbor as yourself, so fulfilling the law of Christ.
*I follow Brian Rosner here. **Likely, these are the words of Christ which Asaph spoke via prosopology.