Again, what business is it of the union what its members believe? More than the obvious. I wasn't aware that Christianity was something you had to sign off on.
They receive money from the SU for providing a service to the student base. Not to start their own little cult.
As far as I can tell, they appear to be trying to cause a conflict for their own gain. PC gone mad and suchlike. Drop the stupid "statement of beliefs" thing, and the problem is gone.