Most Christians like in pick and choose the bible verses to follow and ignore. The Old Testament lays the foundation for the teachings and events found in the New Testament. First, the Old Testament is of great importance to Christians because it is inspired of God (2 Tim. 3.16-17). Some Christians ignore the Old Testament and focus on the New, but only those who read the New in light of the Old can really learn what God is saying in His Word. The Old Testament provides a fore shadow of the blessing revealed in the New Testament (Hebrews 10: 1).
The old testament is very important because The Old Testament Tells the Beginning of Earth and the Human Race, it Records God's Relations with Men in Times Past, Gives Evidences to Support Our Faith, it shows the Evidence that God Exists and there is one true God, it shows the evidence that Bible is the word of God and inspired by God, it shows the evidence the promise of our savior Jesus Christ, in the new testament, it shows the Basic Nature and Needs of Man Have Never Changed, a complete picture of Gods amazing salvation plan is revealed, Proper understanding of the Old Testament helps us understand and appreciate the New Testament, it also shows that the nature of God has never changed. Without OT We would not be able to know what pleases Him and what angers Him and thus build a better relationship with Him. we also know that from the OT that God has always been: good and merciful; holy and just, condemning evil and rewarding good; all-knowing and all-powerful, worthy of praise and worship; faithful to His promises, etc. The Old Testament is also the Word of God just like the New Testament. God speaks to us also through the Old Testament. Without the Old Testament, we would not understand why this Messiah was coming Isaiah 53), and we would not have been able to identify Jesus of Nazareth as the Messiah through the many detailed prophecies that were given concerning Him, e.g., His birthplace (Micah 5:2); His manner of death (Psalm 22:1, 7-8, 14-18; Psalm 69:21), His resurrection (Psalm 16:10), and many more details of His ministry (Isaiah 52:19, 9:2).
God made it very clear that the punishment of sin is death, by using the flood to destroy all life. However after the flood, He promised not to let history repeat itself. Does it mean that man would become sinless that God would never need to bring his wrath on mankind again? Gods anger is not only said in OT it is said even in NT as well .( Acts 5:1-10). Throughout the Old Testament we see God’s judgment and wrath poured out on sin. Likewise, in the New Testament we see that the wrath of God is still “being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness” (Romans 1:18). So, clearly, God is no different in the Old Testament than He is in the New Testament.
There are lots of Old Testament prophecies which were fulfilled in the life of Jesus Christ. Jesus' fulfillment of the Law does not give us license to sin or reason to ignore the Old Testament. He fulfilled the law by being the ultimate sacrifice and now, the ultimate high priest so that our bodies can be temples .Jesus used the Old Testament to show these disciples that He was the Christ the Old Testament foretold (John 3:14, Numbers 21:9). Paul warned those who continue in the Old Law must continue in all of it( Galatians 3:10) .In both testaments, we find all we need for eternal life and godly living (2 Timothy 3:15-17).Both OT and NT are inspired by the Holy spirit(Heb. 10.15-17)
The apostle Peter also affirmed the inspiration of the Old Testament. In 1 Pet. 1.10-11, he said:
Concerning which salvation the prophets sought and searched diligently, who prophesied of the grace that should come unto you: searching what time or what manner of time the Spirit of Christ which was in them did point unto, when it testified beforehand the sufferings of Christ, and the glories that should follow them?
Jesus accepted all OT and NT of it as God's word and that no part of the Bible can be ignored. Jesus continually quoted Old Testament scripture to teach his disciples and criticize the Jewish pharmacies. He ridiculed, “Have you not read that which was spoken to you by God?” (Matthew 22:31) and “Are you not in error because you do not know the Scriptures or the power of God?” (Mark 12:24). He also used stories from the Old Testament to teach, “Have you not read what David did?” (Matthew 12:3).
Jesus believed that the Old Testament was divinely inspired as he referred to the scriptures as “the commandment of God” (Matthew 15:3) and as the “word of god” (Matthew 15:6). He also taught Christians the infallibility of scripture when he stated, “Until Heaven and earth pass away, not the smallest letter or stroke shall pass away from the law, until all is accomplished” (Matthew 5:18) and “the Scripture cannot be broken” (John 10:35).
Jesus also confirmed many of the historical accounts in the Old Testament, such as the destruction of Sodom and the death of Lot’s wife (Matthew 10:15, Luke 17:29-32), the murder of Abel by his brother Cain (Luke 11:51), the calling of Moses (Mark 12:26), the manna given in the wilderness (John 6:31-51), and the judgment upon Tyre, Sidon and Sodom (Mathew 11:20-24).
Jesus even confirmed the most often rejected Old Testament stories, such as the creation story of Adam and Eve (Matthew 19:4-6), Noah and the great flood (Matthew 24:37-39), and the three days of Jonah spent in the belly of a great fish (Matthew 12:40).
Jesus always believed and obeyed the OT scriptures. He never changed the law or never came to destroy the law he only fulfilled.
Some False preachers corrupt the gospel and they teach different gospels, they are more concerned about popularity than purity comforting them in their sin rather than calling them to repentance and They will tell the people what they want to hear.
Also I have seen a horrible thing in the prophets of Jerusalem: they commit adultery and walk in lies; they also strengthen the hands of evildoers, so that no one turns back from his wickedness. All of them are like Sodom to Me, and her inhabitants like Gomorrah” (Jeremiah 23:14).
Beware of false prophets, preachers and priests who tickle the itching ears of evildoers (2 Timothy 4:3,4).
Thus says the Lord of hosts: 'Do not listen to the words of the prophets who prophesy to you. They make you worthless; they speak a vision of their own heart, not from the mouth of the Lord” (Jeremiah 23:16).
Because of the neglect of the false prophets, the people did not repent: “I have not sent these prophets, yet they ran. I have not spoken to them, yet they prophesied. But if they had stood in My counsel, and had caused My people to hear My words, then they would have turned them from their evil way and from the evil of their doings” (Jeremiah 23:21,22).
Jesus being born a man but being also God perfectly fulfilled the law. He Himself had no sin but He obediently paid the price for the curse on man under God's perfect law. Atonement for sin brought into the world by Adam's disobedience was paid for by the obedience of God's Lamb by death on the cross. At the cross He took the curse upon Himself for it is written "cursed is anyone who hangs on a tree" (Gal 3:13). Satan with his wolves in sheep's clothing has corrupted the gospel.
Both Old Testament and New Testament make whole Bible which is God's inspired Word. Because it is inspired and trustworthy, we should read it and apply it to our lives. The Bible is our standard for testing everything else that claims to be true. It is our safeguard against false teaching and our source of guidance for how we should live. It is our only source of knowledge about how we can be saved (2 Timothy 4:16-17).