American author and radio host Joni Eareckson Tada wrote in the book When Is It Right to Die?: Euthanasia on Trial (1993):
       
“And gradually, though no one remembers exactly how it happened, the unthinkable becomes tolerable. And then acceptable. And then legal. And then applaudable.”
     
“GRADUALLY, THE UNTHINKABLE BECOMES TOLERABLE THEN ACCEPTABLE THEN LEGAL THEN PRAISED” (shown on a sign by the Coventry Estates Baptist Church) was posted on Twitter by Peter van der Merwe on May 13, 2019. Both forms of the saying have been printed on many images.
     
     
Wikipedia: Joni Eareckson Tada
Joni Eareckson Tada (born October 15, 1949) is an evangelical Christian author, radio host, and founder of Joni and Friends, an organization “accelerating Christian ministry in the disability community.”
     
Google Books
When Is It Right to Die?:
Euthanasia on Trial
By Joni Eareckson Tada
London, UK: Marshall Pickering
1993
Pg. 63:
And gradually, though no one remembers exactly how it happened, the unthinkable becomes tolerable. And then acceptable. And then legal. And then applaudable.
 
30 October 1996, Canberra (ACT) Times, pg. ?: 
Her name is Joni Eareckson Tada and this is what she says: “Gradually, though no-one remembers exactly how it happened, the unthinkable becomes tolerable and then acceptable and then legal and then applaudable.”
BETTY HEDGECOE
Hawker
 
Twitter
Terri Cook
@MrsCook91
Gradually, though no one realizes how it happened, the once unthinkable becomes tolerable, & then acceptable, & then legal, & then applauded
10:54 PM · Nov 22, 2015·Twitter for Android
   
Twitter
Jim Thomas
@pastorjimtvc
“And gradually, though no one remembers exactly how it happened, the unthinkable becomes tolerable. And then acceptable. And then legal. And then applaudable.”—Joni Eareckson Tada
1:09 PM · Jan 24, 2019·Twitter Web Client
 
Twitter
Peter van der Merwe
@Petervdm896
This applies to what’s happening in South Africa… Gradually the unthinkable becomes tolerable then acceptable then .....
(The following text is shown on a church sign.—ed.)
COVENTRY ESTATES BAPTIST CHURCH
GRADUALLY, THE
UNTHINKABLE BECOMES
TOLERABLE THEN
ACCEPTABLE THEN LEGAL
THEN PRAISED
12:56 PM · May 13, 2019·Twitter for Android
     
Twitter
JoAnn Francis
@JoAnnFrancis9
And gradually,
though no one remembers
exactly how it happened,
the unthinkable becomes tolerable.
And them acceptable.
And then legal.
And then applaudable.
Joni Eareckson Tada
5:29 PM · Dec 22, 2019·Twitter Web App
 
Twitter
Joe Connell
@JPConnell1
And gradually, though no one remembers exactly how it happened, the unthinkable becomes tolerable. And then legal.  And then applaudable.
2:29 PM · Feb 11, 2020 from Dunbar, PA·Twitter for Android
 
Twitter
Matthew Holliday
@Matthew_4_Trump
“Gradually, the unthinkable becomes tolerable, then acceptable, then legal, then praised.”
2:38 PM · Nov 15, 2020·Twitter Web App
 
The Patriot Post
JUN. 25, 2021
Truth
(The following text is shown on a church sign.—ed.)
COVENTRY ESTATES BAPTIST CHURCH
GRADUALLY, THE
UNTHINKABLE BECOMES
TOLERABLE THEN
ACCEPTABLE THEN LEGAL
THEN PRAISED