Because religion is based on faith. And faith is by definition an irrational stand. When religion gets into politics, it simply does not have the same effect as when non-religious gets into the political system. Power is given to people by "god" in a religious political system and this type of thinking allows for a loop that society has trouble breaking.
Yes, it was the priests that were suppose to be the people of "God". Clearly god doesn't care that people are corrupt in his name.
Regardless of the state of Christianity of the time, it was still a religion. Look at the middle east right now. it's in a huge mess.
Yeah, it is the biggest influence. Look at the USA. It's stopping stem cell research, people are still arguing over a women's right to choose, and some parents refuse medical care for their children because it's "against their religion". Completely irrational.
This comes back to the presupposition that reason is necessarily good and faith is necessarily bad. Reason is good. Faith is ba-a-a-d. (Animal farm anyone? I haven't read any orwell in years but somehow he's popped up a few times in this thread)
On the philosophic level, for a starting point, you have to presuppose that your senses and faculties are functioning and showing you an accurate view of reality and a real world around you...and that you aren't insane (Shutter Island) or in a world of someone else's making (Inception) or being completely manipulated (The Matrix).
This isn't a universally held belief: Traditional Buddhists for one don't believe in the real existence of a physical reality. In this one area at least, Buddhists don't depend on a presupposition for their philosophy, while Westerners do.
You also have to presuppose that your intellect and ideas should be used in the world to create and change reality. Again this is not a belief universally held: Eastern practices like Mantras and Trancedental Meditation support the idea that thought should be eliminated to reach a transformation.
I pressume if you are a Westerner who supports rationalism you hold these two presuppositions. Neither of these presuppositions is irrational, but certainly neither presupposition can be proven. You therefore hold them in faith.
Without these and other presuppositions, which were taken and developed by the West from many sources including the Ancient Greeks and especially the Bible, Western Civilization would never have developed.
So, I hope you'll agree, faith is not necessarily a bad thing.
On a practical side, there is no doubt that rational decisions through history have not always been good. Think of Eugenics, the Chinese Cultural Revolution, Stalin's Purges.
Faith-based decisions have produced a lot of good, as I mentioned before.
I find it quite odd you think its irrational to be opposed to abortion. Scientifically human life begins at conception, not at birth. This is the most rational position. Its quite irrational to believe that ending a life while in the womb has nothing morally wrong about it while killing him/her a few months, weeks, days, or seconds later immediately after exiting the birth canal is the worst moral crime. It would be much more rational to make life significant after the baby begins breathing, or after reaching a certain mental ability, or when it can clothe and feed itself.
On that note, why make Infanticide illegal at all? Animals often let the weak one of the litter die afterall. "Survival of the fittest" seems a sensible, rational idea doesn't it? India for one has always practiced infanticide...although it is much more taboo now after colonialism. Surely if we are to be tolorant of other cultures we should not object to this, and perhaps even in solidarity join in the practice ourselves?
Really, I'm having trouble understanding why your particular stance on abortion is rational...can you explain further?