I have another sci-fi short story I cannot remember the title of. It should be about 20 pages long.
It sounds like "Origin" by Timothy Zahn but the length is way off. "Origin" is not "about 20 pages long," it's just over 3 pages in Isaac Asimov's Science Fiction Magazine, February 15, 1982, available at the Internet Archive.
A spaceship with two human beings is blocked by an enormous alien spaceship.
"Greetings," a lilting voice said. "I am Fal-sint. Who and what are you?"
Wordlessly, Rolf handed Kevn the mike. Grimacing, the other cleared his throat. "I'm Kevn Jakoby; my partner is Rolf Sawyer. We're humans, prospecting in this region of space."
"Are you creators or creatures?"
"Uh . . . creatures, I guess."
"Who is your creator?"
Kevn blinked. "No one. We evolved naturally, by ourselves. That's what creature means."
"Not true. If you are creatures then you were created. That is simple definition."
Rolf nudged him. "Ask him what he is," he suggested in a low voice. "Maybe he's got a strange definition for 'creator,' too."
Kevn nodded and asked the question. "I am a creature of the Varka series, designed to serve as a star patroller."
The light dawned. "You're a computer, aren't you? An artificial intelligence." Already Kevn was feeling better about all this. "Well, by that definition we're creators."
Their occupants ask, “Which side of the war are you on?” The humans answer that they do not know what they are talking about, and they were explained that in the Galaxy there is a huge war between Creators and Creatures, robot-like beings.
It seems that the great Conflict is not creators vs. creatures; there are creators and their creatures on both sides:
"You may not yet leave." There was a new note of firmness to Fal-sint's lilt. "If you are creators, I must learn which side of the Conflict you stand on; if you are creatures, I must learn the stand of your creator."
The alien spaceship contains Creatures, so the human replied “we are Creatures too”; the aliens then asked them to prove it.
Kevn took a deep breath, mentally crossing his fingers. "Very well. Fal-sint, you have passed our creator's test." He hoped fervently the other would accept that statement without awkward questions. "We may now admit to be creatures."
"You have proof?" Fal-sint didn't seem angry at the shift in Kevn's story.
Eventually the humans sent them a copy of the Bible, which should have settled the question:
"Yes." Kevn accessed a section of the computer's literature array. "Read here, starting where it says, 'So God created man in His own image.'"
the aliens agreed that the humans were Creatures, but then they added: “Your Creator seems to be really important. Bring us to Him.”
There was a short pause, and Kevn held his breath. Then Fal-sint spoke. "Yes; I see now. Your creator is indeed powerful. I will send word to my creator at once."
Grinning, Kevn winked at Rolf. "Fine. Now, perhaps we creatures can discuss some areas of mutual interest. This Conflict of yours, for instance; and the location of any nearby metal-rich planets."
"I'm afraid not. While we normally would do so, my creator will definitely wish to speak directly to one as powerful as yours. Please communicate with your creator and ask where he would prefer such a meeting take place."
The final catchline was something like “You cannot ever find a priest, when you need one!”
Kevn stared at Rolf. "What do we do now?" he hissed.
Rolf shrugged helplessly. "There's never a priest around when you need one," he muttered.
