There are several reasons why we need apologetics.
The first and most obvious is because we are commanded to defend the faith: 1 Peter 3:15 says, "but sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence."
Second, we need apologetics because it helps Christians know their faith. This is something that is sadly lacking among believers. Most don't know much about their faith, let alone be able to describe the Trinity, the two natures of Christ, His physical resurrection, or even to tell the difference between justification and sanctification. Apologetics helps to define and defend what the truth of the gospel is.
Third, apologetics is an attempt to keep people out of hell. God takes sin very seriously, and He will punish those who have rebelled against Him and are not covered in the blood of Christ. As Christians, we should be motivated to present the truth of salvation in Jesus. We should not sit idly by and ignore the dilemma of the unbeliever. We need to tell them that sin is real because God is real, and that breaking God's law has a consequence. Since we have all sinned, we cannot keep God's law perfectly. Also, we cannot undo the offense to an infinitely holy God because we are not infinite or holy; the only thing left for us is to fall under the judgment of God. But God has provided a way for us to escape that judgment. That is why God became man in Jesus. He claimed to be God, (John 8:24,58; compare with Exodus 3:14). Jesus bore our sins in His body on the cross, (1 Pet. 2:24). By trusting in Christ for the forgiveness of our sins, we will be spared from the rightful judgment of God upon the sinner. Salvation is not found in Buddhism, Islam, relativism, or in one's self: It is only found in Jesus. We need to not only defend God's word and truth, but also present the gospel to all people so they can escape the judgment to come.
Fourth, we need apologetics to counter the bad image that Christianity has received in the media and in culture. Televangelists and their scandals--both sexually and monetarily--are a disgrace to Christianity. The Catholic church hasn't helped with its scandals involving priests. On top of that, the media is very biased against Christianity, and you will see negative opinions of Christianity promoted everywhere.
Fifth, we need apologetics because there is a constant threat of apostasy in the visible Christian church. Such is the case with the Metropolitan Community Church denomination, which openly advocates the support of homosexuality in violation of scripture (Rom. 1:18-32). Also, as of 2002, the Evangelical Lutheran Church is in risk of apostasy by entertaining the idea of accepting homosexual relationships into church. "The United Church of Christ set up a $500,000 scholarship fund for gay and lesbian seminarians Friday and urged wider acceptance of homosexuals by other denominations." (United Church Makes Gay Scholarship, CLEVELAND, June 16, 2000, AP Online via COMTEX). Or "The supreme court of the United Methodist Church was asked Thursday to reconsider the denomination's ban on gay clergy." (Church court of United Methodists asked to decide on gay clergy ban, NASHVILLE, Tennessee, Oct. 25, 2001, AP WorldStream via COMTEX). Such examples are demonstrations of the incredible need for defending biblical truth within those churches that claim to be Christian.
Sixth, another reason we need apologetics is because of the many false teachings out there. Mormonism teaches that God used to be a man on another world, that he brought one of his goddess wives with him to this world, that they produce spirit offspring that are born into human babies, and that you have the potential of becoming a god of your own world. The Jehovah's Witnesses teach that there is no Trinity, that Jesus is Michael the Archangel, that there is no hell, and that only 144,000 people will go to heaven. Atheism denies God's existence, openly attacks Christianity and is gaining ground in public life and schools. Islam teaches that Jesus was not God in flesh, that Jesus did not rise from the dead, and that He did not atone for our sins. It teaches that salvation is partly based on one's works and partly based on Allah's grace. It teaches that the Holy Spirit is the angel Gabriel (Surah 2:97; 16:102); that Jinn are unseen beings created (51:56) from fire (15:27; 55:15); and that Muhammed was greater than Jesus. Even within the Christian church there are false teachings. We can see that from both within the Christian church and outside of it, false teachings are bombarding believers (and nonbelievers) all over the world.
Seventh, the rise of immorality in America is a threat not only to society but also to Christianity. This is a serious issue because an immoral society cannot last long. The Barna Research group statistics show that 64% of adults and 83% of teenagers said moral truth depends on the situation that you are in. 19% of the adult population believes that "the whole idea of sin is outdated." 51% believe that "if a person is generally good, or does enough good, he will earn a place in Heaven."
When a society's morals fail, the society fails. Just look at history and think of Ancient Rome, Ancient Greece or present day Enron, Watergate, and White House interns. Immorality seeps down into all areas of our culture. Consider this: In the New York Times, online, of May 12, 2002, in the article "With Games of Havoc, Men Will Be Boys," the author, Warren St. John, interviews some players of what he says is a very popular video game. One young man says, "What I like to do is get in the car and drive around and do drive-by shootings. You can haul someone out of their car and beat on them and steal their money and their car. It's kind of amusing that you have that ability." .... A publicist from Long Island says the game's allure comes down to "just going on killing sprees." Not all video games are violent, but the fact that it is so popular and that the youth are being trained up by them is very disturbing.
I am not advocating a theocratic socio-political rule administered by stern Christians wearing black-and-white outfits and tall hats. But these kinds of social trends are disturbing, and they reflect a moral decline in America, where what is good is called evil and what is evil is called good. God tells us in Phil. 4:8, Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, let your mind dwell on these things. We cannot ignore God's word without a consequence.
The eighth reason we need apologetics is because schools are not friendly to Christianity. My own experience in non-Christian schools was a strong awakening to the unprovoked hostility that exists in school, where the philosophy teachers, history teacher, and even the art teacher all took shots at Christianity. Don Feder in the Conservative Chronicle, in his article of Sept. 22, 1993, titled "Fighting Censorship, PAW Does it Its Way," said that in some junior high libraries, book titles included The Joy of Gay Sex and How to Make Love to a Single Woman. There is an impressions series for grades one to six which promote the New Age and the occult; a controversial drug education program called Quest, which tells students that they alone can decide whether or not it's OK to use drugs; as well as texts that direct students to fantasize about suicide, attack religion and undermine family authority. Following is an email I received that represents the hostility of secular schools.
Our daughter had acquired an atheist's heart since leaving home and attending college. It seems that the books in college breed atheists because they are full of the philosophy of anti-God thinking. She has been in college for four years now, and one of the last times we had a chance to talk to her, she said that she doesn't think about sin, or heaven, or hell anymore because, according to her, they do not exist. She said that when she was young and asked the Lord to come into her heart, she did not know what she was doing because children do what they are told.