What is the meaning of this slogan?

It means Science is more serious and realistic than religion.

Hippiedom rules okay, even in the petri tray..

See your other question; it means the person expressing that opinion outside a specific forum for discussing it is presenting themselves as smarter than anybody who might see it. That is low class and ignorant outside the forum but would lead to lively debate within that forum. There it would be savaged on the grounds of the existence of a whole realm of religious philosophy, which proves the amount of thought that went into religion. If somebody wore a t-shirt with that slogan in the 200 stacks (Religion in Dewey Decimal) of a library they would look quite the fool.