U.S. President Abraham Lincoln told the Boston (MA) Journal in July 1864:
 
“I have faith in the people. They will not consent to disunion. The danger is, that they are misled. Let them know the truth and the country is safe.”
 
“Let the people know the truth and the country is safe” (cited in print since at least 1906) became the popular version of Lincoln’s remarks. Lincoln’s words were frequently quoted in the 1920s, but the statement quickly lost popularity in the Lincoln quotation canon.
 
A similar Lincoln-themed quotation—but actually the words of Douglas MacArthur in 1944—is “I am a firm believer in the people. If given the truth, they can be depended upon to meet any national crisis.”
 
 
Wikipedia: Abraham Lincoln
Abraham Lincoln (i/ˈeɪbr^hæm ˈlɪŋk^n/; February 12, 1809 – April 15, 1865) was the 16th President of the United States, serving from March 1861 until his assassination in 1865. He led the country through a great constitutional, military and moral crisis — the American Civil War — preserving the Union while ending slavery and promoting economic and financial modernization.
   
29 April 1865, Sandusky (OH) Register, pg. 1:
President Lincoln Had a Presentiment of Death.
A correspondent of the Boston Journal gives an account of a conversation with President Lincoln, from which the following extract will be read with interest:
(...)
The North was tired of the war, and supposed an honorable peace attainable. Mr. Lincoln knew it was not—that any peace at that time would be only disunion. Speaking of it he said: “I have faith in the people. They will not consent to disunion. The danger is, that they are misled. Let them know the truth and the country is safe.”
 
Google Books
Six Months at the White House with Abraham Lincoln:
The story of a picture
By Francis Bicknell Carpenter
New York, NY: Hurd and Houghton
1866
Pg. 263:
“He told me, in July, 1864,” says a correspondent of the Boston “Journal,” “that he was certain he should not oulast the rebellion.”
(...)
“The North was tired of the war, and supposed (Pg. 264—ed.) an honorable peace attainable. Mr. Lincoln knew it was not,—that any peace at that time would be only disunion. Speaking of it he said: ‘I have faith in the people. They will not consent to disunion. The danger is, that they are misled. Let them know the truth and the country is safe.’”
 
Google Books
Memorial Day Annual
By Wisconsin Dept. of Public Instruction
Madison, WI: Democrat Print. Co.
1906
Pg. 48 (Words of Lincoln):
Let the people know the truth and the country is safe.
 
Google Books
The Lincoln Year Book;
Axioms and aphorisms from the great emancipator
Edited by Wallace de Groot Cecil Rice
Chicago, IL: A.C. McClurg & Co.
1907
JUNE
FIRST
Let the people know the truth, and the country is safe.
 
Google Books
Phillips’ Book of Great Thoughts, Funny Sayings:
A stupendous collection of quotes, quips, epigrams, witticisms, and humorous comments : for personal enjoyment and ready reference
By Bob Phillips
Wheaton, IL: Tyndale House
1993
Pg. 278:
Let the people know the truth and the country is safe.
Abraham Lincoln
 
Google Books
The Forbes Book of Business Quotations:
10,000 Thoughts on the Business of Life
Edited by Ted Goodman
New York, NY: Black Dog & Leventhal Publishers : Distributed by Workman Pub. Co.
2006
Pg. 275:
Let the people know the truth and the country is safe.
Abraham Lincoln