> One night he admitted that the professors would not let students going for a PhD get one if they had any religious belief of any kind.

And how would he know? That doesn't seem like the sort of thing a professor would confide to a grad student. It could end the professor's career.

I think it's much more likely that your TA was lying to you in service of his own political agenda.

In any case, it is manifestly untrue that this is widespread because half of all working biologists profess to hold religious beliefs.