Why do people believe the Old Testament is null and void?
God is consistent. He does not change. When He says something, you can live by it, whether it is said in the Old Testament or the New Testament. Yes God is dealing with people through grace instead of the Law, but He has not changed. Paul tells us that the Law was never meant to justify people but rather to bring people to Christ:
(Galatians 3:23-25 NIV) {23} Before this faith came, we were held prisoners by the law, locked up until faith should be revealed. {24} So the law was put in charge to lead us to Christ that we might be justified by faith. {25} Now that faith has come, we are no longer under the supervision of the law.
Now there are lots of things in the Old Testament that don't really pertain to us. Such as the instructions for building the Tabernacle, or even the Law of Moses. However, people again stray when they think they can reject what the Old Testament says out of hand. For example people will say that we do not live under the Law, we live under grace. Well that is true, but what does it mean? People will try to tell you that it means we don't have to be concerned about things like the 10 commandments any longer because that is the Law and we are not under the law anymore. Well these same people might be surprised to find that the New Testament confirms all but one of those 10 commandments. The only one which is not confirmed for us by a new commandment in the New Testament is to keep the Sabbath.
You might wonder why the commandment to honor the Sabbath was not included. I think you can find your answer in the Old Testament: (Exodus 31:15-17 NIV) {15} For six days, work is to be done, but the seventh day is a Sabbath of rest, holy to the LORD. Whoever does any work on the Sabbath day must be put to death. {16} The Israelites are to observe the Sabbath, celebrating it for the generations to come as a lasting covenant. {17} It will be a sign between me and the Israelites forever, for in six days the LORD made the heavens and the earth, and on the seventh day he abstained from work and rested. It was to be a lasting covenant between God and the Israelites. I am not saying that we shouldn't keep the Sabbath holy, but it is not commanded to us in the New Testament.
Now look at a few passages of Scripture with that in mind: (Hebrews 10:26-29 NIV) {26} If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, {27} but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. {28} Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. {29} How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace? This is a warning to those who think that they can continue to sin after salvation.
Paul also warns us about the same thing: (Galatians 5:19-21 NIV) {19} The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; {20} idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions {21} and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.
Now look at what God said in the Old Testament, and you will see that He is very consistent: (Ezekiel 18:20-24 NIV) {20} The soul who sins is the one who will die. The son will not share the guilt of the father, nor will the father share the guilt of the son. The righteousness of the righteous man will be credited to him, and the wickedness of the wicked will be charged against him.
{21} "But if a wicked man turns away from all the sins he has committed and keeps all my decrees and does what is just and right, he will surely live; he will not die. {22} None of the offenses he has committed will be remembered against him. Because of the righteous things he has done, he will live. {23} Do I take any pleasure in the death of the wicked? declares the Sovereign LORD. Rather, am I not pleased when they turn from their ways and live?
{24} "But if a righteous man turns from his righteousness and commits sin and does the same detestable things the wicked man does, will he live? None of the righteous things he has done will be remembered. Because of the unfaithfulness he is guilty of and because of the sins he has committed, he will die.
Alexander my dear, I ask questions for the opinions of others. You are not obligated to answer if you don't want to. Thanks and have a blessed day.
And no, the Old Testament is NOT null and void. Read the scriptures again.