Just noticed these

1. Maybe the street people lived around the church for a while. And as It's time of awaking was drawing closer. It started to influence the street people to It's will.

2. Not all of the were scientists in the field of physics. Lisa was there to translate the book. Susan was there for carbon dating. Even if they had a working knowledge of basic physics. They might have not known what a tachyon particle is.

And Brian explaining what a tachyon is was for the viewer. But he couldn't break the 4th wall and tell us.

3. Someone has to be the slacker of the group.

1. I believe it was said that the PoD(even in the form of the green goo prior to possessing Kelly) could control 'simple minded organisms'. It was also said that the street people were likely suffering from 'chronic schizophrenia'(Kelly says this to Walter not too soon after everyone arrives at the church) meaning that their thought processes were much less complex compared to a normal human's because they have a routine they repeat over and over. Also, being schizophrenic means they are likely used to hearing and obeying 'voices in their head'. This is how it is explained they are susceptible to the PoD's influence.
2. The other poster is correct. At least half of the people in the church were non-physics scientists(chemist, radiologist, etc.) with one priest and one ancient language translator as well. And, as the other poster said, this excuse allowed Brian to be Mr. Exposition to the audience itself.
3. I think Walter is actually supposed to be the comic relief character. For example, I think he is similar to Shaggy and Scooby in that, while everyone else is running around trying to figure out the "mystery", he's basically letting his fear make him run around like a child scared by a boogeyman.

I would like, if I may, to take you on a strange journey.

4. Both Catherine and Brian made a fool out of Thomas Magnum, the famous Hawaiian private eye.