Historically, large majorities of the American citizens  identify as being Christian in their religious faith.  Often at 90% levels.

America's Constitutional Government requires God-Fearing leaders to operate in the best interests of the people.

Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other. 

- John Adams Address. October 11, 1798

God-Fearing means that one recognizes God the Creator, and the promise of Godly judgement upon death.  Therefore leaders will attempt to follow God's Will during their earthly life.  This includes protecting the God-given rights of citizens Life, Liberty and the Pursuit of Happiness.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.

-Preamble to the Declaration of Independence

Benjamin Franklin credited God with influencing the Convention used to create the Constitution:

 “I can hardly conceive a transaction of such monumentous importance.... without being in some degree influenced, guided, and governed by that omnipotent, omnipresent, and beneficient Ruler in whom all inferior spirits live and move and have their being.”  (1788 - TRBF p343)

The Constitutional text states the signing date using Year of our Lord, which of course can only mean Jesus Christ.  This was common among the Christian nations of Europe and the soon to be nation of the United States of America.

Constitution: Article VII:

The Constitution was completed and sent to the states for ratification "by the Unanimous Consent of the States present the Seventeenth Day of September in the Year of our Lord one thousand seven hundred and Eighty seven"

Secularists have in recent years attempted to remove Christianity from most aspects of US Government.    Separation of Church and State is their battlecry.

Of course these words do not appear in the Constitution.  Freedom of Religion is enshrined in the 1st Amendment.