“Our lives being to end the day we become silent about things that matter” is a popular saying that has been printed on many posters and credited to Martin Luther King, Jr. (1929-1968). However, the saying has been credited to him in print since only about 1996.
 
“Our lives begin to end the day we become silent about things that matter” has been cited in print since at least 1994, when it was not credited to any author.
 
   
Wikipedia: Martin Luther King, Jr.
Martin Luther King, Jr. (January 15, 1929 – April 4, 1968) was an American pastor, activist, humanitarian, and leader in the African-American Civil Rights Movement. He is best known for his role in the advancement of civil rights using nonviolent civil disobedience based on his Christian beliefs.
   
Wikiquote: Talk:Martin Luther King, Jr.
Unsourced
Our lives begin to end the day we become silent about things that matter.
   
MLKDay.gov
Rev. Dr. Martin Luther King, Jr. Quotes
(...)
. Our lives begin to end the day we become silent about things that matter.
 
Google Books
It Starts on the Frontline
National School Public Relations Association
1993 (This Google Books date could be incorrect.—ed.)
Pg. ?:
“Our lives begin to end the day we become silent about things that matter.”
Martin Luther King, Jr.
   
2 February 1994, Rockford (IL) Register Star, pg. 5B, col. 1:
MEDITATION OF THE DAY: Our lives begin to end the day we become silent about things that matter.
 
4 September 1994, Cedar Rapids (IA) Gazette, pg. 6E, col. 3 real estate classified ad:
Our lives begin to end the day we become silent about things that matter.
 
Google Books
The Spiritual Dance of Life:
Where Two Worlds Meet
By M. Teri Daunter
Petoskey, MI: Mobius Pub.
1995
Pg. 21:
Our lives begin to end
About the day we become silent
About the things that matter.
Author Unknown
 
Google Books
Best of the Word for Today
By Bob Gass
Alachua, FL: Bridge-Logos
1996
Pg. 23:
Martin Luther King Jr. said, “Our lives begin to end the day we are silent about things that matter.”
 
Google Books
If You Only Knew
By Thomas G. Martin
Glendale, CA: Griffin Pub.
1997
Pg. 164:
Our lives begin to end the day we become silent about the things that matter.
Martin Luther King, Jr.
 
17 February 1998, The Capital (Annapolis, MD), “Zamora Found Guilty” (AP), pg. A1, col. 6:
He concluded his argument by quoting slain civil rights leader Martin Luther King Jr.
 
“Dr. King said, ‘Our lives begin to end the day we become silent about things that matter,”” Mr. Parrlsli said.
 
9 April 1999, Brownsville (TX) Herald, “Thirty-THree Cent Forum,” pg. A10, col. 5:
The late Dr. Martin Luther King Jr. once said that our lives begin to end the day we become silent about things that matter.
 
Hathi Trust Digital Library
Public Papers of the Presidents of the United States
Administration of William J. Clinton
2000-2001
Washington, DC: United States Government Printing Office
2001
Pg. 798:
Remarks at the NAACP Fight for Freedom Fund Dinner in Detroit, Michigan
April 30, 2000
Pg. 799:
Dr. King once said that our lives begin to end the day we become silent about things that matter.