Why do people get defensive when you challenge their beliefs? This applies to Christians, atheists, etc. The reason why people are threatened and get defensive when you challenge their beliefs is because they do not really know what they believe or why they believe it. For many people (maybe even most), they believe what they believe simply because they were told to; this is how they were raised, this is what they were taught. They have never bothered to think about it for themselves and to make it their own. This is imperative with the Christian faith, because God does not have any grandchildren; you cannot ride the coat tails of your parents.
When someone challenges my beliefs, I do not get defensive. In fact, I do not even get offended. If someone cannot have a serious, mature, respectful, intelligent, rational, honest conversation with me, if they want to attempt to resort to mocking my faith (for example), I will meet them exactly where they are at (save for cussing). I will mock them in return, in order to grab their attention and make them listen, show them just how silly their beliefs are by comparison, and then attempt to return us to a civil conversation.
What do those around you think of you or of Christians in general? Do they think Christians are thoughtful, patient, gracious, and love their opponents, despite the disagreement? Or do they think something else? Is it Christ-like for professing Christians to boo non-Christians and to tell them they are going to Hell? This is the behaviour of the immature. How did Jesus treat sinners? Jesus commanded Christians to love their enemies. What does Paul say? He says that if you do not have love, then you are nothing!
Many Christians not only need to learn what they believe and why they believe it, but they also need to learn to argue. Years ago I used to post on a site called ex-Christian.net. Most of the so-called "Christians" on there could not refute the atheists they encountered, and would childishly resort to calling them names and telling them they were going to Hell. I received a personal e-mail from the owner of the site asking me to stop posting because I was such a thorn in the side of these supposed ex-"Christians." I turned the tables on them and they could not answer me. I even wrote a letter to these supposed "Christians" on behalf of the site telling them that if they do not know how to respond, keep their mouth shut, watch those who do, and learn. Had I known better, I would have taken the owner's private e-mail to me and posted it publicly on the site for all to see. Hind sight.
I am not concerned whether they believe what I believe (though I wish they would), nor is it my responsibility to convert them. My responsibility is to speak the truth, to plant and water. God gives the increase. No matter how intelligent I am, how great an orator I may be, how persuasive my arguments may be, or how well I might deliver the truth, I will never be able to convert a single soul. It is God's responsibility to convert people—not mine. Christians need to learn this.
You do not always need to be "nice" and "polite," and Christians should never be acquiescing, but you do need to be compassionate and loving. Be thoughtful, patient, and as gracious as possible. If you pick any moment from Jesus' life, you are witnessing genuine love in all its forms. God is love, and Jesus is God, ergo everything Jesus said and did was enveloped with love. When He spoke with the Pharisees, He told them exactly what they needed to hear. That was loving. He did not sugar coat it. He did not water it down. He did not pander to them. When you do these things, you make Christianity look just as bad as what you see from the folks at Westboro Baptist Church. There is nothing wrong with the use of mocking, sarcasm, satire, wit, or even being a bit abrasive, if it is done with genuine love. If you are doing it just to be an arse, then, yeah, you have a problem.
If you feel threatened by non-Christians (or anyone really) when they challenge you, and you have to resort to childish ad hominem attacks, you should probably pause and reflect on your own character (or lack thereof). I learned long ago when I was young that people's attacks are not really against me, but against the Lord; I am merely the conduit through which they are attempting to direct them. Sadly, there are many people in leadership positions who have this same immaturity when dealing with their opponents. This speaks volumes about their church, because such people should not be in such positions. Learn what you believe, Christian, and why you believe it, and then you will not feel so threatened when others challenge your world view.