June 14, 1777
Was the day that the Second Continental Congress adopted the 13 stars and stripes flag designed by Betsy Ross.

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 Pledge of allegiance was written for the children's magazine Youth's Companion by Christian Socialist author and Baptist minister Francis Bellamy on September 7, 1892.

Francis Bellamy's original Pledge:
I Pledge allegiance to my Flag, and to the Republic for which it stands: one Nation indivisible, With Liberty and Justice for all.

More about the Pledge
In 1923 the National Flag Conference called for the words my Flag to be changed to the Flag of the United States. The reason given was to ensure that immigrants knew to which flag reference was being made. The words "of America" were added in 1924. The United States' Congress officially recognized the Pledge as the official national pledge on December 28, 1945.

In New York City on April 22, 1951, the Board of Directors of the Knights of Columbus adopted a resolution to amend their recitation of Pledge of Allegiance at the opening of each of the meetings of the 800 Fourth Degree Assemblies of the Knights of Columbus by addition of the words "under God" after the words "one nation."

Four score and seven years ago our fathers brought forth on this continent a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
Now we are engaged in a great civil war, testing whether that nation, or any nation, so conceived and so dedicated, can long endure. We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this.

Lincoln's Gettysburg Address mentioned "under God" as well...
But, in a larger sense, we can not dedicate—we can not consecrate—we can not hallow—this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here. It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us — that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion — that we here highly resolve that these dead shall not have died in vain — that this nation, under God, shall have a new birth of freedom — and that government of the people, by the people, for the people, shall not perish from the earth.