What is Science?
Science is the search for objective truth. How do we know if something is objectively true? By testing it.
If it cannot be tested, then it is NOT science.
Objective truth is important because we know objective truth is actually true. Religious truth has no credibility. If religious truth had any value for finding what is actually true then we would allow religious testing into areas of life where it is currently banned, such as our drug testing methodology. In America, new drugs have to be rigorously tested for safety and efficacy, usually in scientific double-blind clinical testing. Would any religious wacko be willing to take drugs that bypassed all this scientific testing and were merely approved by a religious committee which prayed for guidance from god to make FDA rulings? It's painfully obvious that all the religious leaders in the world praying to god for guidance would NOT be able to predict the safety and efficacy of a new drug any more reliably than a 5-year old doing eeny-meeny-miny-moe.
If we didn't use objectivity to verify the truth of our ideas our medical doctors would still be using leeches to bleed sick people because they "believed" in the technique and didn't bother to test their theory.
This is why "intelligent design" cannot be called science, because it cannot be tested. If it cannot be tested then it simply has no credibility and doesn't deserve further consideration. Anybody could sit down for one day and dream up 500 different hypothetical supernatural origins for the universe and none of those 500 hypothesis would be worth investigating without at least a shred of evidence to begin with. In fact, that's exactly where religion came from - superstitious, ignorant men who believed the earth was flat sitting around several thousand years ago dreaming up fantasies.
Here is a graphic example of the difference between science and religion. Imagine you wake up and find yourself isolated in a white room. When you went to sleep everything was normal and you have no idea how you got into the white room. The religious approach would be to curl up in a ball in the corner of the room and pray. The truly devout would accept the new condition as god's will and simply wait for god to make his next move. The scientific approach would be to start thinking and reasoning. The rational person would reason that he isn't dead yet so someone must have put him in the room for a purpose. He would hypothesize that someone might be monitoring him and he would shout to see if he could get some attention. He might bang on the walls to see what they're made of and and test for an escape route. We could go on and on but, hopefully, you get the point. The religious approach is to ignore reality and just believe whatever you want to believe but the scientific (aka rational, logical, objective) approach is to think and reason and try to understand the world and perform tests and experiments to verify if our understanding is correct.
But what if we find out that gods really do exist? If any gods would reveal their existence then all rational people would believe in gods. Suppose the gods revealed themselves to us and proved to us they have unlimited supernatural powers. Then rational people would believe in the existence of all-powerful gods and supernatural power (actually, it would then be considered natural, not supernatural). But the next question would be this, do these gods deserve our respect? An objective, unbrainwashed reading of the christian bible shows that the god of christianity is a vile, filthy, evil god who deserves to be killed, not worshipped. The god of christianity is filthy because the christian myth was created by filthy-minded, superstitious, ignorant men who believed the earth was flat.
At this point in time, the existence of god is just like the existence of space aliens - there may be a lot of people who believe but there simply isn't a speck of evidence. A few centuries ago most people believed the earth was flat. Clearly, millions of people believing something does not make it true. (Actually, belief in gods is far less credible than belief in aliens. We see ourselves on this planet so it's reasonable to think there might be other creatures on other planets. But we have never seen any evidence for the existence of a supernatural creature so there is no basis to suppose that any supernatural creature has ever existed at all.)
Is there any supernatural claim in any religion that can be objectively tested and shown to be true? No. Not even one. Therefore, believing in god is unscientific because it is a belief that can't be tested and for which there is no evidence.
See also: Intelligent Design Has Missing Link