"If we ever forget that we're one nation under…

U.S. President Ronald Reagan said at a prayer breakfast in Dallas on August 23, 1984:

“Without God, there is no virtue, because there’s no prompting of the conscience. Without God, we’re mired in the material, that flat world that tells us only what the senses perceive. Without God, there is a coarsening of the society. And without God, democracy will not and cannot long endure. If we ever forget that we’re one nation under God, then we will be a nation gone under.”

Reagan was referring to the “one Nation under God” line in the Pledge of Allegiance. “If we ever forget that we’re one nation under God, then we will be a nation gone under” became one of Reagan’s most popular quotes.

Wikipedia: Pledge of Allegiance
According to the United States Flag Code, the Pledge of Allegiance reads:

I pledge allegiance to the Flag of the United States of America, and to the Republic for which it stands, one Nation under God, indivisible, with liberty and justice for all.
(...)
The phrase “under God” was incorporated into the Pledge of Allegiance June 14, 1954, by a Joint Resolution of Congress amending §4 of the Flag Code enacted in 1942.

On October 6, 1954 the National Executive Committee of the American Legion adopted a resolution, first approved by the Illinois American Legion Convention in August 1954, which formally recognized the Knights of Columbus for having initiated and brought forward the amendment to the Pledge of Allegiance.

Wikipedia: Ronald Reagan
Ronald Wilson Reagan (/ˈrɒnəld ˈwɪlsən ˈreɪɡən/; February 6, 1911 – June 5, 2004) was the 40th President of the United States (1981–1989). Prior to his presidency, he served as the 33rd Governor of California (1967–1975), and was a radio, film and television actor.

American Rhetoric
Ronald Reagan
Remarks at a Dallas Ecumenical Prayer Breakfast
delivered 23 August 1984 at Reunion Arena, Dallas, TX
(...)
Without God, there is no virtue, because there’s no prompting of the conscience. Without God, we’re mired in the material, that flat world that tells us only what the senses perceive. Without God, there is a coarsening of the society. And without God, democracy will not and cannot long endure. If we ever forget that we’re one nation under God, then we will be a nation gone under.

24 August 1984, Rockford (IL) Register Star, “Reagan mixes religion with politics” by Bob Minzesheimer (Gannett News Service), pg. 8A, cols. 2-5:
DALLAS—(...) Thursday at a non-denominational prayer breakfast that took on the trappings of a political rally, President Reagan told 17,000 cheering persons that “religion needs defenders against those who are only for the interests of the state.”
(...)
Reagan concluded by saying, “without God there is no virtue” and that “if we ever forget that we are one nation under God, then we will be a nation gone under ... May God keep you, and may we, all of us, continue to keep God.”

First Amendment Religion Clauses
SATURDAY, OCTOBER 25, 2008
One Nation Under God - Ronald Reagan
At a prayer breakfast on August 23, 1984, President Ronald Reagan said,

“You know, if we look back through history to all those great civilizations, those great nations that rose up to even world dominance and then deteriorated, declined, and fell, we find they all had one thing in common. One of the significant forerunners of their fall was their turning away from their God. Without God, there is no virtue, because there’s no prompting of the conscience. Without God, we’re mired in the material, that flat world that tells us only what the senses perceive. Without God, there is a coarsening of the society. And without God, democracy will not and cannot long endure.

“If we ever forget that we’re one nation under God, then we will be a nation gone under.”

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Austin Shuey
‏@AustinShuey
“If we ever forget that we are One Nation Under God, then we will be a nation gone under.” ~ Ronald Reagan
9:53 PM - 12 Apr 2014