“To err is human; to forgive, divine” is from Alexander Pope’s An Essay on Criticism (1711) . “To eat is human; to digest, divine” is a food parody.
 
“To eat is human. To digest is divine” was published in The Nettleton Cook Book (1909). Harvard professor and poet Charles Townsend Copeland (1860-1952) has been credited since at least the 1930s, but there is no evidence that he coined the saying. American novelist Mark Twain (1835-1910) has been credited since the 1990s, but there is no evidence that he ever said it.
 
“To eat is human; to drink wine, divine” is a related saying.
 
   
Wiktionary: to err is human
Etymology
From Latin errāre hūmānum est.
Proverb
to err is human
1. Everyone makes mistakes.
Usage notes
The phrase is often used as a part of a longer phrase in English, “to err is human; to forgive, divine” (Alexander Pope, “Essay on Criticism”).
 
HathiTrust Digital Library
The Nettleton Cook Book
By Grace L. Lawrence; George H. Nettleton Home Association, Kansas City, Mo.
Kansas City, MO: Little Craft and Stationery Shop
1909
Pg. 138:
TO EAT IS HUMAN.
TO DIGEST IS DIVINE!
 
1 October 1912, Adrian (MI) Daily Telegram, “Bixby Hospital Will Get $268.85,” pg. 5, col. 5:
Roll call was answered on this occasion with favorite recipes, which dovetailed nicely the sentiment of the evening: “To eat is human, to digest divine.”
 
April 1913, Health, “Foods and Feeding” by Leon Patrick, pg. 52, col. 1:
“To eat is human, to digest divine.”
 
Chronicling America
23 April 1915, The Missourian Magazine, “Etiquette for Men,” pg. 6, cols. 2-3:
As soon as your fellow-workers have assembled, spring some new jokes like, “To eat is human, to digest—divine.”
     
Google Books
The Macmillan book of Proverbs, Maxims, and Famous Phrases
By Burton Egbert Stevenson
New York, NY: Macmillan
1948
Pg. 575:
To eat is human, to digest divine.
CHARLES T. COPELAND, Epigram. (c. 1925)
 
Google Books
3 January 1955, Life magazine, pg. 72, col. 2:
“TO eat is human; to digest, divine,” the eminent educator, Charles Townsend Copeland, once observed, and from all-night hamburger joint to soigne pressed-duck salon millions of too-well-fed U.S. citizens would chorus agreement.
 
Google Books
Words of Wellness:
A Treasury of Quotations for Well-Being
Edited by Joseph Sutton
Published by author
1991
Pg. ?:
To eat is human; to digest, divine. —Charles Townsend Copeland
 
Google Books
And I Quote:
The Definitive Collection of Quotes, Sayings, and Jokes for the Contemporary Speechmaker
By Ashton Applewhite, William R. Evans III and Andrew Frothingham
New York, NY: St. Martin’s Press
1992
Pg. 198:
To eat is human, to digest, divine. — Mark Twain
 
Google Books
Forbes Book of Quotations:
10,000 Thoughts on the Business of Life
Edited by Ted Goodman
New York, NY: Hachette Books
2016
Pg. ?:
To eat is human; to digest, divine.
Charles Townsend Copeland