First, I want to preface this by saying that my hostility and disrespect for Christianity is not based on any experiences with Christians. Almost all of my experiences with Christians have been very respectful. I have nothing against Christians per se, and if someone decides to be a Christian of his own free will then I accept that (even though I wouldn't support it).
That being said, here are the biggest reasons why I have no respect for the ideology and worldview of Christianity:
1. I disrespect Christianity because it is a Big Lie. The Big Lie was a technique attributed to Hitler, by which one repeats a lie with a gigantic scope over and over until it becomes commonplace. The lies that "God exists," "God created the universe," "God created man," "Jesus existed," "Jesus was fully man and fully God," are all so nonsensical, much like any other fairy tale. But because they are given a privileged position in our society, one would not dare to say that Christianity is a lie on the same order as any other fairy tale peddled as truth (Santa Claus, tooth fairy, etc)
2. I disrespect Christianity because it is absurd. Nothing in Christianity makes sense. God is outside the universe, but thinks and acts like a material being ("he" even gets mad!). Jesus is fully man and fully divine. God, Jesus and the Holy Spirit are three beings that are one that are three. Jesus was executed so he would change his own mind about a rule he made. Adam and Eve are condemned for an act for which they could not possibly have any moral responsibility. If you think about anything in the Bible for more than ten seconds, it makes absolutely no sense. That's where the occult aspect of Christianity comes in: they say you need to "have faith" and you will read these absurd stories with "new eyes."
3. I disrespect Christianity because it is immoral. As someone once said, anyone who followed the OT would be considered a murderer: anyone who followed the NT would be considered insane. This blood-soaked book, filled with stories of murder and genocide of the most horrendous immorality, is not spiritual and it is not moral. And the Jesus stories are equally immoral, although not because of their violence but because of the immoral principles that they invoke (there are exceptions, such as the parable of the stoning, although even there the moral principle underlying Jesus' actions is dubious at best).
4. I disrespect Christianity because it goes against the fundamental principles of justice. This is probably the single clearest example of why Christianity deserves to be stamped out. In Christianity, "salvation" is achieved by believing in Jesus as saviour. Then you get to go to Heaven. If you refuse to believe, then you suffer. This is what Christians believe justice is. Not judging people by their fruits, but by their belief or lack thereof in a lie and a fiction. By any reasonable human standards, there are millions of morally upright people, even heroes, who are condemned by Christianity for not believing in Jesus. Christianity rejects everything that is good in human beings in the name of being a good Christian believer: therefore it stands against everyone, everywhere, even Christians (although of course Christians believe that they are worthless sinners, and so have no good in them apart from Christianity- this is how they justify hating morally good atheists).
5. I disrespect Christianity because it contradicts the very essence of human existence. If Christianity is true, then our lives have no purpose, controlled by a being that we cannot understand and a plan hidden from us. If Christianity is true, then nothing else can be true, since God (or indeed any powerful transcendent agent) can make it so that 2+2=3, that the Earth does not rotate in 24 hours, or that murder is good (the last two having been done in the Bible). If Christianity is true, then we do not have free will (as, once again, proven in the Bible). If Christianity is true, then concepts are invalid (because they are predicated on induction, and induction is predicated on a uniform universe). If Christianity is true, then any meaning we find in the things around us is pure invention, because there is no truth and concepts are a sham.
In short, to be Christian contradicts what it means to be human.
When do I disrespect Christians? I disrespect Christians when they try to enforce a culture of tolerence for Christianity, and a culture of intolerence against every other worldview, morality or creed. Christianity should not be tolerated. Christianity should not be allowed to state its case without objections. It is a primitive, unjust, tribal ideology that needs to be stamped out from public discourse.
I also disrespect Christians when they show their hypocrisy through their actions. And no, I do not mean televangelists or pastors who grub for money. I mean Christians who, for instance, cry at funerals (which contradicts believing in an ever-benevolent afterlife), or commit crimes (which contradicts the belief in always being watched by an omniscient creature that judges you).
You can read more from Francois Tremblay at his blog Check Your Premises .