Apparently, when I was a toddler, a large and probably over-friendly dog jumped at me and terrified me. It took years for me to learn to like dogs again.
Others have more painful experiences: perhaps they were knocked over, or bitten by a dog. Perhaps they knew someone who was ravaged by a dog. Perhaps they had early experience of highly-trained guard dogs who would attack strangers. Some of them may never really come to accept dogs as (mostly) friendly, tame pets.
Fear or distrust of dogs is so common that when we hear someone insisting they hate dogs, we assume they've had a bad experience with them in the past. We're sympathetic, on the whole. We might make gentle enquiries about why they feel that way. We might share our own similar experiences. We might show them photos of soft and cuddly dogs that could hardly fail to make them smile. Or we might just leave the topic alone.
What we don't do is to start heavy arguments about why dogs are wonderful, or tell them they should come with us to the local stray dogs' home. We don't give them heavy books about why a dog is man's best friend, or try to convince them of the pointlessness of having gerbils or hamsters as pets.
So.... why do so many Christians take a heavy line with people who attack the church, or their personal beliefs? Chances are that someone who is suspicious of Christianity has been injured in some way in the past by Christians. Perhaps they had a rigid and dogmatic upbringing, heavily disciplined for any infringement of Christian values. Perhaps they were spurned at a church they visited for dressing in scruffy jeans. Perhaps they've come across hypocritical Christians who say one thing and do another.
John Piper says:
Let us learn to discern whether the words spoken against us or against God or against the truth are merely for the wind--spoken not from the soul, but from the sore. If they are for the wind, let us wait in silence and not reprove. Restoring the soul, not reproving the sore, is the aim of our love.(from "A Godward Life")
How right he is. When other people attack us, or our faith, or the church, or even God, it does no good at all to argue. Yes, we need to be sure what we believe and be prepared to explain it. But we need to do so from the perspective of the person we're talking to. Jesus told us that the entire law is summed up in the commands to love God, and to love our neighbours as ourselves. Loving our neighbour doesn't just involve doing practical things for them. It can mean listening to them at a deep level, hearing where they're coming from, understanding why they've formed the views they have. Maybe at some point we can introduce them to Jesus, but we're unlikely to to have any effect until we've shown them honest friendship and shown that we take them seriously.