To pretend the Founders were not Christians is anti-truth and makes you no better than the Texan book-writers.
That's a straw man argument.
This issue is not whether or not some or all of the Founding Fathers were Christians. The issue is the claim that the United States was founded according to Christian biblical precepts and thus its laws should reflect these beliefs. This claim is an outright history denying lie perpetrated by Christian Nationalists. This lie is easily revealed by reading what our Founding Fathers had to say about religion and government.
Some examples:
Benjamin Franklin: "When a religion is good, I conceive it will support itself; and when it does not support itself, and God does not take care to support it so that its professors are obliged to call for help of the civil power, ‘tis a sign, I apprehend, of its being a bad one."
John Adams: "It will never be pretended that any persons employed in that service had interviews with the gods, or were in any degree under the influence of Heaven, more than those at work upon ships or houses, or laboring in merchandise or agriculture; it will forever be acknowledged that these governments were contrived merely by the use of reason and the senses."
James Maddison: "Because Religion be exempt from the authority of the Society at large, still less can it be subject to that of the Legislative Body. The latter are but the creatures and vicegerents of the former. Their jurisdiction is both derivative and limited: it is limited with regard to the co-ordinate departments, more necessarily is it limited with regard to the constituents. The preservation of a free Government requires not merely, that the metes and bounds which separate each department of power be invariably maintained; but more especially that neither of them be suffered to overleap the great Barrier which defends the rights of the people. The Rulers who are guilty of such an encroachment, exceed the commission from which they derive their authority, and are Tyrants. The People who submit to it are governed by laws made neither by themselves nor by an authority derived from them, and are slaves."
Do some research and you'll find more of the same. Thomas Jefferson had a lot to say about the subject as well.
Here is another way to look back. In 1797 the Treaty of Tripoli was signed and unanimously ratified by the Senate. It contains the words "As the government of the United States of America is not, in any sense, founded on the Christian Religion..." and you would think that a "Christian Nation" would be upset by this clear statement. The complete text of this treaty and the news of its signing was published in several newspapers of the day and yet there is no evidence of any public outcry or backlash.
This nonsense about America being a Christian nation is revisionist history perpetrated by Christian Nationalists in an attempt to subvert the constitution and the clearly articulated intentions of the Founders of the United States. Of course, don't take my word for it, do some research of your own.