The late John Gerstner gave the following reasons for apologetics:
1. People who argue against arguments (That is, Christians who claim that apologetics are unseemly, reason is unreliable, and only unquestioning faith is virtuous) are, in fact, making arguments. They are using their heads to justify not using their heads. To provide reasons for not using reason is simply not very smart.
2. You will encounter those who will, as they should, ask why. You need a because that is more substantive than just because.
3. When sane people appear to be against reason, they actually are not. When Tertullian said he believed (in God) because it was absurd (as opposed to logical) he was in fact saying that it was logical that the ways of an infinite, Holy God should (by reason) appear absurd to fallen creatures.
4. If Christianity claims to be true, then it requires prove. If we only needed to claim truth, the Christianity would be established, as would Mormonism, Scientology, Islam, and all other religions. Proof is not just for the atheist, but also the believer. As Chillingsworth put it:
I am certain that God has given us our reason to discern between truth and falsehood, and he who makes no use of it, but believes things he knows not why, I say, it is by chance that he believes the truth and not by choice; and I cannot but fear that God will not accept the sacrifice of fools.Even when we jettison reason in favor of experience, we are actually reasoning. The very primitive reason is this: I have had an experience, and that experience could only come from God. But this reasoning is very weak, and requires the listener to take the speaker’s word for it. The apologist who has only experience is in a position of extreme weakness, like the Moody Bible student who witnessed about Christ in her life to a University of Chicago professor. The scholar, through probing questions that she could not begin to answer, eventually had her doubting her own salvation. She was right and he was wrong, but she didn't know her apologetics. She had the proof but didn't know how to express it, and ultimately believed she didn't have it.
5. Christ proved He was who He claimed to be.
Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the miracles themselves. (John 14:11)Before healing the paralytic, Jesus forgave him of his sins, thus claiming His divinity. He then did not say: believe it or not. Rather he went on to prove His divinity by means that no rational person could deny.
But so that you may know that the Son of Man has authority on earth to forgive sins...." Then he said to the paralytic, "Get up, take your mat and go home." (Matt. 9:6)
6. The bible testifies to its own inspiration, but not through circular reasoning. The gospels have proven historically reliable, and they testify to a miracle working Jesus, miracles of which His enemies do not deny but rather attempt to attribute to Satan.
7. Through apologetics we demonstrate that the Creator is God, that God certifies His Son, that His Son certifies the Word, and that the Word certifies the gospel.