This question and topic generally today refers to Christianity. The word apologetics is somewhat misleading and has nothing to do with apologizing in the modern sense of the word. Apologetics is building the case for explaining and defending the Christian faith. So, the straightforward answer is that apologetics defines, “What Christians Believe and Why Christians Believe It.”
First, an examination of the ‘What is Believed’ includes knowledge about God, creation, nature, origins of man, sin, morality, stewardship, life’s purpose, Jesus Christ, redemption, salvation, Heaven, Hell, the Christian church, the Bible, the Old Testament history of nations, End Times and eternity, for starters.
Then there is the second part of apologetics, ‘Why Christians Believe It’. Christians start with a concept that the Bible is the inspired word of God and therefore totally reliable as truth about what it contains. They trust the evidence that proves the Bible is historically correct, that it is the best documented of all ancient books and that it contains prophecy that is 100% accurate as only God could see the future of human history as if it is already completed. The Bible is a miracle in its own right as it was written over a period of 1600 years by 40 authors in complete harmony and without any contradictions.
Christians trust and believe the evidence of both Biblical and secular records, Josephus AD 75 & Tacitus AD 116, of the life, teaching, miracles, death by crucifixion, resurrection (witnessed by over 500 people) and ascension to heaven of Jesus Christ. They further believe the evidence of the dramatically transformed lives of Jesus 12 disciples who boldly proclaimed the truth of the gospel and were martyred for their testimonies. The fact is that no one willingly dies or is martyred for a lie. Then there is the miraculous contribution of Christianity to Western Civilization; literature, art, science, music, mathematics, medicine, education, philosophy, law and much more.
Finally, there is the witness and testimonies of the hundreds of millions of followers of Jesus Christ through the centuries. Why so many believers? The answer is because Jesus is the only way to eternal life and a transformed life full of meaning and purpose. He is ‘The Way’ to receive God’s gifts of joy, love, peace, forgiveness, contentment, faith and eternal hope.