This Nation wasn't created by Muslims, Buddhists, Hindus, Jews, or Communist Atheists. It was created by Christians. That's the reality. So if you love America, you do owe Christians a sizable debt of gratitude. However if you hate America, you probably see that much differently.
But regardless, without Christians there would be no United States of America. So for at least one day of the year, they should be shown respect and gratitude. All Americans owe them that much.
Your repetitious argument suffers from a glaring flaw, allow me to illustrate.
This nation was founded by white British males so we should continue to honor all British men in America by extending to them a place of power to exert permanent control our national dialog.
Our nation was founded by slave owners so we should set aside a special day to honor slave owners for using their property to help build our nation.
End of examples.
You see the problem here? Even if our founders were devout Calvinists, as many claim, that does not mean anything today. Their faith or lack of faith means no more than their British citizenship or their ownership of slaves or any other quality or flaw they had. If the nation really wanted religious zealots to control our social dialog they would have control but they do not and there is a reason for that: People do not like religious zealot hypocrites telling them what to do and sitting in judgement of their sins.