The founders were most certainly explicitly Christian and while many expressed support for 'freedom of religion' - that more or less meant freedom to choose whatever Christian sect you please. But even that is rather dubious, considering one of the main grievances of the founders was the Quebec Act - which gave freedom to Catholics in the Quebec province - which at the time included a big portion of what is now the USA.
The Maryland Constitution of 1776 said "XXXIII. That, as it is the duty of every man to worship God in such manner as he thinks most acceptable to him; all persons, professing the Christian religion, are equally entitled to protection in their religious liberty"
"XXXV. That no other test or qualification ought to be required, on admission to any office of trust or profit, than such oath of support and fidelity to this State, and such oath of office, as shall be directed by this Convention or the Legislature of this State, and a declaration of a belief in the Christian religion.'
New Jersey restricted political office to Protestants in their 1776 constitution
"XIX. That there shall be no establishment of any one
religious sect in this Province, in preference to another; and
that no Protestant inhabitant of this Colony shall be denied the
enjoyment of any civil right, merely on account of his religious
principles; but that all persons, professing a belief in the faith
of any Protestant sect, who shall demean themselves peaceably
under the government, as hereby established, shall be capable of
being elected into any office of profit or trust, or being a
member of either branch of the Legislature, and shall fully and
freely enjoy every privilege and immunity, enjoyed by others their
fellow subjects."
The Delaware constitution of 1776 mandated that all office holders take an Oath to the Trinity.
"ART. 22. Every person who shall be chosen a member of either house, or appointed to any office or place of trust, before taking his seat, or entering upon the execution of his office, shall take the following oath, or affirmation, if conscientiously scrupulous of taking an oath, to wit:
" I, A B. will bear true allegiance to the Delaware State, submit to its constitution and laws, and do no act wittingly whereby the freedom thereof may be prejudiced."
And also make and subscribe the following declaration, to wit:
" I, A B. do profess faith in God the Father, and in Jesus Christ His only Son, and in the Holy Ghost, one God, blessed for evermore; and I do acknowledge the holy scriptures of the Old and New Testament to be given by divine inspiration.""
Other states did similar things.