The argument (at least the smart argument) isn't that all the founders were not Christians - that is clearly not correct but that there were many founders who did not want a Christian government for various reasons including not being Christians and/or fearing the results of a theocracy.
What we can say is that they were accepting of public expressions of faith and thus the banning of Christmas trees or voluntary prayer is not justified.
Virtually every original state required its representatives to take a Christian oath.
Massachusetts Constitution
Chapter VI. Oaths and Subscriptions; Incompatibility of and Exclusion from Offices; Pecuniary Qualifications; Commissions; Writs; Confirmation of Laws; Habeas Corpus; The Enacting Style; Continuance of Officers; Provision for a future Revisal of the Constitution, etc.
Art I.--Any person chosen Governor, Lieutenant-Governor, Counsellor, Senator, or Representative, and accepting the trust, shall, before he proceed to execute the duties of his place or office, make and subscribe the following declaration, viz.--
"I, A. B. do declare, that I believe the christian religion, and have a firm persuasion of its truth; and that I am seized and possessed, in my own right, of the property required by the Constitution as one qualification for the office or place to which I am elected."