As we Americans gear up for two of the most important holidays of the year, Thanksgiving and Christmas, it is important to ask why a holiday like Thanksgiving should be so important to our nation so many years, beginning in 1621. We all know the story of the Pilgrim's first Thanksgiving in New England the second year of their residence there, as they celebrated the great harvest God had given them (they had landed in November the previous year, had no time to prepare for winter, and had lost half their population through starvation and disease).
This idea, that God is the source of our prosperity, continued very strongly in America, and our first president, George Washington declared an official national day of thanksgiving during his first year in office. Later, Abraham Lincoln set Thanksgiving Day for the Thursday of the last full week in November. We today need to understand that this official day of thanksgiving has its roots in the conviction that our prosperity is the direct gift of our creator God. In fact, the founding fathers of the United States based their whole theory of government on the fact that "all men are created equal, and that they are endowed by their creator with certain inalienable rights." The rights of "life, liberty and the pursuit of happiness" are not the creatures of government to be granted or withheld as government wills, but come directly to each person from the hand of their creator. Thus a day of thanksgiving is very much bound up with the roots of the United States as a nation. We are not a nation made up of one or two ethnic groups, but our unity goes right back to our creator and the rights He has given equally to all of us.
A few nations have tried to repeat the success of the United States, even copying much of our Constitution, but have always failed because they based their unity not on God the creator, but on human factors such as ethnic identity. This is interesting because the founders of the United States claimed that the truths about all men being created equal and having god-given rights to life, liberty and the pursuit of happiness are so obvious that they are "self-evident." Self-evident means not only that everybody ought to see them, but also that they are impossible to contradict. Now our founding Fathers were declaring their principles over against the British system where every person born or living in the country was a subject of the King, but not a free citizen with inalienable rights. But our founders also believed that their self-evident truths are the only basis for legitimate government anywhere because for them the purpose of government is to "secure these rights" of liberty to the people.
From all of this we can see how the principles on which the United States was founded are bound up with the practice of thanksgiving to God for our prosperity. Without the creator God, there is no sense in talking about Thanksgiving. But without the creator God the principles on which our nation is based are also made void. Atheists don't found nations like the United States.