“I can’t believe it’s not Buddha!” is a pun on “I Can’t Believe It’s Not Butter!” that has been printed on many images. There have been several jokes.
   
“A Catholic saw the face of Jesus in a tub of margarine.  His neighbour from Nepal saw it and said ‘I can’t believe it’s not Buddha’!” was posted on Twitter by Queen Allen on July 20, 2010. “An Islamic man found the face of Allah in a tub of margarine.His neighbour from Nepal saw it and said ‘I can’t believe it’s not Buddha’!” was posted on Twitter by Butafly on July 7, 2011. “A Tibetan monk sees the face of Jesus in a tub of margarine. He raises his eyes to the heavens and exclaims ‘I can’t believe it’s not Buddha!’” was posted on Reddit—Jokes on July 4, 2020.
   
“Did you hear about the new low-fat religion? ‘I Can’t Believe It’s Not Buddha‘“ is a related joke.
 
   
Wikipedia: I Can’t Believe It’s Not Butter!
I Can’t Believe It’s Not Butter! is a spread brand produced by Upfield and marketed as a substitute for butter.
 
Wikipedia: Gautama Buddha
The Buddha (also known as Siddhattha Gotama or Siddhārtha Gautama) was a philosopher, mendicant, meditator, spiritual teacher, and religious leader who lived in Ancient India (c. 5th to 4th century BCE). He is revered as the founder of the world religion of Buddhism.
     
Twitter
Queen Allen
@re9al
A Catholic saw the face of Jesus in a tub of margarine.  His neighbour from Nepal saw it and said “I can’t believe it’s not Buddha”!
7:31 AM · Jul 20, 2010·Ping.fm
   
Twitter
Hridaya
@thikey
RT @SoundMatt: A Cathlic saw d face of Jesus in a tub of margarine.  His Nepali neighbour saw it and said “I can’t believe it’s not Buddha”.
3:41 PM · Jul 20, 2010·TweetDeck
 
Twitter
Martha Hummingbird
@pinkangelpoo
A Priest+a Hindu are making Toast.Priest says ‘‘Look!Theres an image of Christ in my Margerine’‘Hindu says ‘‘I Cant believe its not Budda’‘
12:38 PM · Mar 5, 2011·Twitter Web Client
 
Twitter
Butafly
@xbutaflyx
An Islamic man found the face of Allah in a tub of margarine.His neighbour from Nepal saw it and said “I can’t believe it’s not Buddha”!
12:44 PM · Jul 7, 2011·Twitter for iPhone
 
Google Books
The Mammoth Book of One-Liners
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2012
Pg. ?:
A Muslim found the face of Allah in a tub of margarine. His neighbour from Nepal saw it and said: “I can’t believe it’s not Buddha.”
   
Twitter
Bob Kostic
@causticbob
I called my Tibetan friend over and said. “Look, there’s the face of Jesus in my butter!” he replied: “I can’t believe it’s not Buddha.”
6:11 PM · Feb 24, 2012·TweetDeck
 
Twitter
Claire
@aclaireytale
A priest & a Hindu make toast. The priest says “theres an image of Jesus in my margarine!” The Hindu replies “I cant believe its not Budda!”
6:27 AM · Jun 7, 2012·Twitter Web Client
   
Twitter
Bob Kostic
@causticbob
An Islamic man found the face of Allah in a tub of margarine.  His neighbour from Nepal saw it and said “I can’t believe it’s not Buddha”
5:18 PM · Nov 12, 2013·Twitter Web Client
 
Twitter
Bob Kostic
@causticbob
An Islamic man found the face of Allah in a tub of margarine. 
His neighbour from Nepal saw it and said “I can’t believe it’s not Buddha”
7:55 AM · Mar 12, 2018·Twitter Web Client
 
Twitter
tis me. Nic
@tismenic70
Was spreading some marg on my toast this morning… Can you believe it? .. The face of Jesus appeared..
It thought.. I can’t believe it’s not Buddha….
#wednesdaywit
2:13 AM · Jun 19, 2019·Twitter for Android
 
Twitter
Dad Jokes
@Dadsaysjokes
Have you all heard about the monk who claimed to see the face of jesus in a tub of margarine?
He said: “I can’t believe it’s not Buddha.”
3:45 PM · Dec 20, 2019·Twitter for iPhone
     
Twitter
Dad Jokes
@Dadsaysjokes
I opened a tub of soft margarine and it looked just like the face of Jesus!
I showed my Thai friend and he said: “I can’t believe it’s not Buddha.”
5:01 PM · May 28, 2020·Twitter for iPhone
 
Reddit—Jokes
Posted by u/RipKipley July 4, 2020
A Tibetan monk sees the face of Jesus in a tub of margarine.
He raises his eyes to the heavens and exclaims “I can’t believe it’s not Buddha!”