First of all, I want to assure you that we are commanded to answer the questions of those who ask us why we believe what we believe. 1 Peter 3:15 states, "But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear." We are to be ready to answer the questions we receive about our faith. In fact, there is an entire field of Bible studies meant to answer these questions. It is called the field of apologetics and it deals with answering the objections to Christian faith and doctrine.
However, that does not mean that we have to have all knowledge and be quick to answer any objection to Christianity. No one automatically has the answers to every question. One of the best ways to learn to answer these questions is to seek an answer when someone challenges you with a question you had not considered before. Tell them that you do not have a good answer right now but that you will seek to find one for them. And then do just that. You can seek the advice and counsel of others, search the scriptures, and pray for God to show you how to answer their question. The entire process is a way that you can grow in the things of the Lord.
The question you have been asked is a complex one and could be drawn out easily to book length. However, I am sure you want something simpler than that. You may find another approach that is helpful to you, but I will give you an approach I might take if asked this question.
I might initially respond to their question with this question: "What is your authority for truth?" If they did not understand my meaning (as is likely the case), I would explain to them that the world is full of conflicting authorities that all claim to be telling the truth. In order to determine what is true in any given case, each person has to decide who or what he will believe. When two authorities conflict, the one that I accept over the other is my authority for truth. The authority I take above all others is my final authority for truth. Here are some of the authorities people have for truth with scriptures that describe them:
- Tradition: the authority of mans long-time practice (Matthew 15:1-9, esp.v.3,6,9; Colossians 2:8)
- Philosophy: the authority of mans greatest wisdom (Colossians 2:8; Acts 17:18, with v.16,22,23)
- Science: the authority of observable facts and repeatable physical laws (1 Timothy 6:20)
- Scholarship: the authority of academic learning and agreed upon knowledge (2 Timothy 3:7; Isaiah 29:9-12)
- Pragmatism: the authority of what brings the desired results (1 Corinthians 1:18-25)
- Experience: the authority of personal experience (Romans 10:2-3)
I would challenge the person asking the question to figure out what their authority for truth is. Then, I would tell them that my authority for truth is the teaching of the word of God--the Bible. I would declare the word of God as the only God-given authority for truth (John 8:31-32; 17:17). I would explain that there are many evidences supporting the authority of God's word. These evidences include fulfilled prophecy, influence of the Bible in individuals and in world history, scientifically accurate statements in scripture, the endurance of the Bible through the ages, and others. If the person wanted to go further, it would open the door to have Bible studies with them and see what God would do in their heart. If they reject it outright, then I have still given them an answer.
Also, I would push them to define their final authority for truth. Then, I would point out that their authority for truth has flaws as well. Science and philosophy are continually changing. Experience differs from one person to another. The others have their flaws too. This way, we answer their questions by both defending what we believe and challenging their beliefs. Learning to answer objections to the faith is a lifelong process, but perhaps this will be a good start.