The books of the Old Testament were composed by people living under the Old Covenant, the Law of Moses. It is important to keep that context in mind when studying them. Seek first to understand the intentions of the author in his time and place rather than reading into them meanings from the New Testament age or later.
Take for example Psalm 35:9: “Then my soul will rejoice in the LORD and delight in his salvation (NIV) or “be happy of his deliverance (NET).” The word “salvation” to most Christians calls up the blessings accomplished by Jesus Christ on the cross: forgiveness of sins, justification, eternal life. That, however, is not what David had in mind. “Salvation” in the Psalms nearly always refers to a temporal deliverance from trouble or danger. Verses 1-3 of the psalm explain the situation:
“O Lord, fight those who fight with me! Attack those who attack me! Grab your small shield and large shield, and rise up to help me! Use your spear and lance against those who chase me! Assure me with these words: ‘I am your deliverer!’” (Psalm 35:1-3)
The concept of God as “Judge” also has a different connotation. Most people today think of God’s judgment as something to be feared and avoided, and rightly so! However, the Psalms frequently present God’s arising for judgment as something earnestly desired. For example:
“Let the sky rejoice, and the earth be happy! Let the sea and everything in it shout! Let the fields and everything in them celebrate! Then let the trees of the forest shout with joy before the Lord, for he comes! For he comes to judge the earth! He judges the world fairly, and the nations in accordance with his justice.” (Psalm 96:11-13)
The reason for the difference in perspective: a “judge” in the Old Testament period was not pictured as a black-robed magistrate on a bench in a criminal court but more like a hero or knight in shining armor, one who defended justice and the cause of the poor and defenseless. C. S. Lewis wrote:
“The ancient Jews, like ourselves, think of God’s judgment in terms of an earthly court of justice. The difference is that the Christian pictures the case to be tried as a criminal case with himself in the dock; the Jew pictures it as a civil case with himself as the plaintiff. The one hopes for acquittal, or rather for pardon; the other hopes for a resounding triumph with heavy damages.” (C. S. Lewis, Reflections on the Psalms)
When Jesus Christ died on the cross, He brought to a close the age of the Old Covenant, the Law of Moses, and simultaneously inaugurated the New Covenant in which we live. In every age, a person can only be accepted by God by His grace through faith, so that will be consistent. God’s method of managing His people, however, can be different, so how one’s faith is expressed and lived out can differ as well.
The most obvious difference to a reader will be the centrality of the Temple and sacrificial system of worship. The Temple represented the presence of God dwelling among His chosen people, Israel. There the priests represented the people to God, and sacrificial offerings were the prime way to publicly express worship, repentance, and thanksgiving. Superficial people often fell into believing they could “buy off” God through offering sacrifices without heart (Ps. 50:7-13). On the other hand, those like David who possessed insight and sincerity always knew that God wanted the worshiper’s heart first:
“Certainly you do not want a sacrifice, or else I would offer it; you do not desire a burnt sacrifice. The sacrifices God desires are a humble spirit – O God, a humble and repentant heart you will not reject.” (Psalm 51:16,17).
Then, where one’s heart was right, sacrifices could be acceptable to God as an expression of inner faith:
“Then you will accept the proper sacrifices, burnt sacrifices and whole offerings; then bulls will be sacrificed on your altar.” (Psalm 51:19)
While we no longer express worship to God through animal sacrifices, His desire from us is the same. He still desires the hearts of His people above all else.
Some theological differences are reflected in the Old Testament books due to their pre-cross perspective. David prays after his sin with Bathsheba, “Do not reject me! Do not take your Holy Spirit away from me!” (Psalm 51:11). Do Christians need to fear that God will withdraw His Spirit from them because of sin? No. The ministry of the Holy Spirit in Old Testament times was different. Today, every Christian receives the permanent indwelling of the Holy Spirit at the moment he or she trusts in Jesus Christ. Before Christ came, however, the Holy Spirit came only upon certain individuals to empower them for special service (such as prophets or kings), and there was no promise of permanence. In that psalm, David is actually praying that God will not take away his anointed role as king of Israel as He actually had done to the previous king, Saul (1 Samuel 16:14). While disobedient Christians may face temporal consequences or the discipline of God because of sin, they do not need to fear that God will take away His Spirit, because He has promised, “I will never leave you and I will never abandon you.” (Hebrews 13:5).
Forgiveness of sins also appears slightly different. Under the Law, the key concept was atonement, which means a “covering” for sin. Forgiveness was indeed offered by a gracious God to those who trusted in His lovingkindness, but it was at best temporary and up-to-date. Nowhere in the Law of Moses is there offered forgiveness for tomorrow’s sins. However, in the New Testament we find an emphasis on Jesus Christ’s finished work on the cross. Where the Law offered a temporary “covering” for sins, Jesus Christ became “the Lamb of God, who takes away the sin of the world” (John 1:29). While “for the blood of bulls and goats cannot take away sins” (Heb. 10:4), now “by one offering [Christ] he has perfected for all time those who are made holy” (Heb. 10:14). The Christian has the blessing and privilege of rejoicing in the fullness of acceptance accomplished for us by our Savior Jesus Christ, “who became for us wisdom from God, and righteousness and sanctification and redemption” (1 Cor. 1:30).
Therefore, while reading the Old Testament books you want to read first to obtain accurate understanding of what their authors meant. But then you want to apply their great truths in the light of fully developed New Testament revelation by faith in Jesus Christ.
About 1700 years after God created everything, He sent judgment on a rebellious race through a worldwide Flood. He later separated the nations with different languages and scattered them from Babel. Abraham, Isaac, and Jacob were founding fathers of the Hebrew people. Sold into slavery, Joseph became a powerful foreign leader. The Israelites developed into a great nation for ~400 years in Egypt, until their deliverance from bondage. Then Moses took the people across the Red Sea and taught them God’s Law at Mt. Sinai. Joshua led the Israelites into the Promised Land after a 40-year trek in the wilderness because of unbelief.
During the transition toward monarchy, there were 40 deliverer-rulers called “Judges,” the last of whom was Samuel. The first three Hebrew kings—Saul, David, and Solomon—each ruled 40 years. Under Rehoboam, the Hebrew nation divided into northern and southern kingdoms, respectively called Israel and Judah. Prophets warned against worshipping the foreign god Baal. After the reign of 19 wicked kings in the north, Assyria conquered and scattered the northern kingdom. In the south, 20 kings ruled for ~350 years, until Babylon took the people into captivity for 70 years. While Zerubbabel, Ezra, and Nehemiah led the Jews back into Jerusalem over a 100-year period, Esther was a savior-queen in Persia. More than 400 “silent years” spanned the gap between Malachi and Matthew.