Each sentence evokes a different story.
in the abovementioned "Microcosmic God".
They are creatures under a forcefield, but just the one species of bioengineered creatures in their enclosed region, not a galaxy.
Observers are somehow able to couple with individuals on various worlds to learn about the subject's civilization
in
"The Short Ones" by Raymond E.Banks.
The inability to recall this story was a crazymaker until it was answered on these boards.
Also not a galaxy, but small electronic manufactured people that can be interacted with, to test how politician candidates would handle real-life citizens' issues.
... the creation of a single, physically scaled down galaxy inside some kind of force field
is in
"Fessenden's Worlds" by Edmond Hamilton.
At last a story with... a real galaxy! In microscopic scale. It has been so long since I read it that I don't remember if it is also time-accelerated or simply microscopic scale.
Fessenden created the tiny universe, then observes it as he destroys worlds in it.
Maybe he observed in accelerated time, but could also have just blasted entire worlds and watched the results in real time.
I don't recall lifeforms and cultures being observed having any religions, though, just the mad destructive scientist.
The only story I recall in which the isolated organisms
is
"Sandkings" by George RR Martin.
All of these are short stories or novellas, though, not novels. And though each sentence is evocative of a different story, none of these stories match the entire description.
