The Expositor's Bible Commentary notes how much the Old Testament permeated the thinking and writing of the New Testament authors: "One very notable feature of the N[ew] T[estament] is the extent to which it alludes to or quotes the O[ld] T[estament]... to provide proof of statements made, confirmation for positions espoused, illustration of principles advanced, and answers to questions raised....It is apparent that the NT writers and our Lord himself were so steeped in the language and truths of OT revelation that they naturally expressed themselves in terms reminiscent of it" (ibid.).

The simplest way to understand how the Old Testament applies to Christians under the New Covenant is to study what the Apostles taught on the subject.

James authored his epistle sometime before he was martyred in A.D. 62. As the half brother of Jesus Christ (Matthew 13:55), he was familiar with Jesus' approach toward the Old Testament and God's laws. James refers to that law as "the royal law" in James 2:8, quoting Leviticus 19:18. He then goes on to refer to it as "the law of liberty" in James 2:12, recognizing that obedience to that law frees us from sin and its harmful consequences (James 1:25).

James taught that simply saying we have faith and believe in God is useless (James 2:19). He uses the Old Testament examples of Abraham and Rahab to show our faith must be accompanied by actions—that faith without works is dead (James 2:17-26). He also points out it isn't enough to avoid sin. If we know to do good, but don't do it, that also is sin (James 4:17), as Jesus Christ taught in the Sermon on the Mount (Matthew 5:17-48).

The Apostle Peter also uses the Old Testament as his authority. His two epistles were apparently written in the 60s before he was martyred in A.D. 67 or 68. In his first epistle, Peter quotes from the Old Testament more than a dozen times as the authority for what he is saying. He repeats God's command in Leviticus 11:44, telling us to "be holy in all your conduct, because it is written [in the Old Testament Scriptures], 'Be holy, for I am holy'" (1 Peter 1:15-16). Quoting Isaiah 40:8, he reminds us that "the word of the Lord endures forever" (1 Peter 1:25).

In his second epistle, written shortly before his death (2 Peter 1:14-15), Peter reminds us the Old Testament prophets spoke (and wrote) under the inspiration of God's Holy Spirit (2 Peter 1:20). He exhorts us to "be mindful of the words which were spoken before by the holy prophets" in the Old Testament, as well as the words of the Apostles (2 Peter 3:1-2).

The Apostle John repeatedly emphasized the need to keep God's commandments in his epistles, apparently written between A.D. 85-95 when he was the last of the original 12 apostles still living. "Now by this we know that we know Him, if we keep His commandments. He who says, 'I know Him,' and does not keep His commandments, is a liar, and the truth is not in him" (1 John 2:3-4). See also I John 5:3.

The Apostle Paul wrote to the evangelist Timothy, "All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness…" (2 Timothy 3:16-17). Since Paul defined "Scripture" in the previous verse (verse 15) as that which Timothy had known "from childhood," this could refer only to the Old Testament—since the New Testament had not yet been written.

Yet many theologians see Paul as teaching the Old Testament scriptures are no longer authoritative for Christians, distorting some of Paul's difficult-to-understand passages to support their claim that Jesus Christ—by dying on the cross—abolished the Old Testament law. They ignore Peter's cautionary warning that "Paul, according to the wisdom given to him, has written…some things hard to understand, which untaught and unstable people twist to their own destruction…." (2 Peter 3:15-16).

When we examine Paul's writings carefully, it is clear he consistently appeals to the Old Testament scriptures as the main authority for what he taught. The first accusations Paul was disregarding God's law came from certain Jews who objected to his teaching that gentiles could be saved without submitting to the rite of circumcision. They falsely accused him of abandoning God's law and his Jewish heritage, although Paul clearly stated ” …I worship the God of my fathers, believing all things which are written in the Law and in the Prophets." (Acts 24:14) — the Law and in the Prophets being a Jewish term for the entire Old Testament.

Those same inaccurate and slanderous rumors that started with Paul's false accusers so long ago are still circulating today, having become the basis of what is now commonly referred to as "Pauline theology," which presents Paul as someone committed to separating Christianity from its Jewish roots. It portrays him as initiating changes that repudiate Old Testament laws. But, throughout his life Paul defended Old Testament scripture as not only inspired, but also profitable for "instruction in righteousness" for all Christians (2 Timothy 3:15-17).

The ceremonial aspects of the Sinai Covenant, which included laws of offerings and sacrifices and other duties of the priesthood, ceased when the second Temple was destroyed in 70AD. However all the principles and laws of the Old Testament have validity for understanding and in many cases for the direct application of godly principles for New Covenant Christians. The record from these men is clear. They uphold the Old Testament as God's inspired revelation to mankind for all time and affirm that keeping God's commandments remains a requirement for Christians today. It is little wonder, therefore, that Paul stated, "…I would not have known what sin was except through the law…" (Romans 7:7).