"Name of the Snake" by R. A. Lafferty (ISFDB, Wikipedia, Great Science Fiction & Fantasy Works), first published in Worlds of Tomorrow, April 1964, available at the Internet Archive.
A Christian missionary travels to a planet populated by humans (or human-looking aliens) to spread the good word.
The encyclical was titled modestly "Euntes Ergo Docete Omnes": "Going therefore Teach Ye All."
[. . .] It was in the implementation of the command that Padreco Barnaby was now on that remote planet, Analos.
Could one call the Analoi humans? Had their skeletal remains been discovered on old Earth, they would unhesitatingly have been classed as human. The oddly formed ears—not really as large as they seemed—somewhat Gothic in their steepled upsweep, their slight caudal appendage, their remarkable facial mobility and chameleon-like complexions, these could not have been read from their bone remains. But how are we to say that their ears are more grotesque than our own? When did you last look at your own ears objectively? Are they not odd things to be sticking on the sides of a person's head?
The natives welcome him and show him their advanced society. They claim to have eliminated all vices, are completely free from sin, and therefore not in need of salvation.
"But we haven't any sins. That's the whole point about us. We've long since passed beyond that. You humans are still awkward and guilt-ridden. You are of a species which as yet has no adult form. Vicariously we may be the adult form of yourselves. The idea of sin is an aspect of your early awkwardness."
They show him around in a public computer driven car; one particular point was that the seats in the car all face towards the middle, because everyone on the planet is friends with each other.
The Padreco hailed a taxi. A taxi is a circle. That is to say that one clambers over and sits in the single circular seat that faces inward. The Analoi are gregarious and like to gaze on the faces of their fellows. Only the shame-capable humans would wish to sit in unfacing rows. The driver sits above in an open turret, and dangles his head down to talk.
Along the route the missionary smells smoke. Surprised he asks whether the natives still use wood fire. 'Only for certain ceremonial functions' is the answer.
Padreco Barnaby raised his head.
"I smell wood burning," he said suddenly. "You no longer use wood for fuel here."
"In one case only," said Landmaster. "An ancient and seldom employed ritual of ours."
The missionary is unconvinced that the natives are free from sin. After witnessing things like public suicide booths, finally he concludes that while they do not have any of the old sins, they have invented new ones. They are, in his words, the most horrible of all sinners, those who give 'a new name to the snake'.
"I will record it here. You practice infanticide, juvenicide, senectucide, suicide."
"Yes, the Gentle Terminators."
"You murder your own children who do not measure up to your atrocious norm."
"You have invented new lusts and perversions.
"There are the evil who are evil openly. There are the evil who hide their evil and deny that they are venomous. There are the ultimate in evil who keep the venom and change the Name of the Snake."
The natives are very happy about this, since they take great pride in being the best at whatever they do. They then reveal that the ceremonial functions they needed a fire for is heating a big cooking pot to put the missionary in. They are the descendants of a cannibal tribe, and their traditional way of dealing with missionaries is eating them.
"We revert, little priest. In this one case we revert. It is our ancient answer to the obstreperous missioner who persists in asking us the irksome question. We cannot allow ourselves to be irked."
Padreco Barnaby couldn't believe it. Even after they put him in the monstrous kettle he couldn't believe it. They were setting the long tables for the feast—and surely it was all a mistake!
The story ends with the natives asking the missionary about his favorite 'cannibals-eating-a-missionary' caricature punchline, while he is being cooked.
"Shoes and all, little priest. We like the flavor. What was your own favorite caption for the race-memory cartoons, Padreco?
"You can't do this to me!"
"Yes, that was a good one. But it was the subscript, as I remember it, and the caption was 'Famous Last Words.' However, my own favorite, while it concerns anthropophagi, does not concern a missioner. It was the cannibal chief who said, 'My wife makes a fine soup. I'll miss her.' What was your favorite of the kettle jokes, Shareshuffler?"
I think the name of the story was "The Name of the Snake", but I could not find anything with that title.
Google is not your friend. You should have done an ISFDB title search.
I think I read this story in some sort of anthology, about 15 years ago. The story was very short, a few pages at most.
Probably in the Lafferty collection Nine Hundred Grandmothers; maybe you recall one of the other stories from the table of contents. "Name of the Snake" is 10 1/2 pages in my copy.