> (If not, why does this same mechanism work for the AGW hypothesis?)

Because your comparison is flawed in multiple ways.

> Another unscientific claim is that all scientists agree with global warming, therefor it is scientifically "true" that it is happening.

This is something of a straw man. I have no doubt that there are people out there making this claim...just as there are people out there who think unicorns exist. It does nothing to refute this claim because only a small kooky minority are making it.

The REAL point is that "Most experts in the field believe it is true, therefore it is reasonable to go with that belief until we have a reason to think otherwise".

You are confusing "scientific proofs" with how one should act when faced with making a decision. We cannot say that "AGW is true because So-and-so says it is"....that is an argument from authority. However, as citizens trying to make decisions about how to run our society, it's reasonable to go along with the consensus of (expert) scientists in matters of science.

This brings up another point, it's not a matter of what scientists as a whole statistically believe... it's a function of what field the scientists are in, their experience, and their expertise. The assertion is that most experts in the field of climate science (and related fields) agree with AGW and that it is a somewhat concerning issue (though the issue of to what degree is in debate), and there is a sort of rippling of agreement throughout semi-related fields which is worthy of consideration but less heavily weighted.

Where your point is fatally flawed is you are making a false comparison. You cannot compare Christianity which is a vague cultural identity with what they have found via research.

I think if you talk to the majority of scientists who are Christian, you will find that they are Christian mostly only in culture. They are not fundamentalists. They believe there is something called God, and there was this guy a few thousand years ago with some good ideas and maybe he was connected with this God....but if you press them I think you will find they are generally flexible and admit they don't know for sure and are just going on belief.

What you are talking about is an UNSCIENTIFIC BELIEF that they have arrived at completely arbitrarily. It's the same as if you found out that the majority of scientists prefer chocolate ice cream....that doesn't lend some validity to chocolate...it's just an interesting statistic.

AGW on the other hand IS a scientific idea. It's something that was arrived at THROUGH A SCIENTIFIC PROCESS. The fact that most experts agree with it (if it's true) lends it credibility because they either:

a) were able to reproduce the results with their process
b) studied the process of others and found them reasonable

You can argue that the science is flawed, or argue that most experts DON'T agree with it...but it's perfectly reasonable and correct to think that something is more likely to be true if the experts say it is in proportion with the amount of consensus...when you are talking about science . The logical fallacy is only if you claim it is DEFINITELY true, particularly based on a small amount of consensus.

The opinion of scientists absolutely has weight....in matters of science. Christianity as a whole is entirely outside the realm of science, and therefore the fact that many scientists happen to be Christian is irrelevant.

Furthermore, if you do a survey of all the scientists in the world, I think you may find that the majority of them are not Christian. However, the consensus for AGW and it's potential harm will remain.