That is a valid reason for the 1st Amendment, but which does not mean the gov. cannot favor one general faith, which it cannot escape doing unless it imagines that its precepts and principals do not have a religious basis, and that the Founders and the people did not operate out of one general faith in opposition to another.
In his discussion of freedom of religion in his monumental Commentaries on the Constitution of the United States, Father of American Jurisprudence Joseph Story clarified the meaning of the First Amendment with regard to the priority of Christianity stated:
..the real object of the [First] amendment was not to countenance, much less to advance Mahometanism, or Judaism, or infidelity by prostrating Christianity; but to exclude all rivalry among Christian sects and to prevent any national ecclesiastical establishment which should give to a hierarchy the exclusive patronage of the national government (1833, 3:728, emp. added).
James Iredell, a U.S. Supreme Court judge appointed by George Washington, articulated this point succinctly in 1788 in the debates on the wording of the Constitution:
But it is objected that the people of America may perhaps choose representatives who have no religion at all, and that pagans and Mahometans may be admitted into offices . But it is never to be supposed that the people of America will trust their dearest rights to persons who have no religion at all, or a religion materially different from their own (Elliot, 1836, 4:194).
Samuel Johnston, governor of North Carolina and member of the Constitution ratifying convention in 1788:
Those who are Mahometans, or any others who are not professors of the Christian religion, can never be elected to the office of President or other high office, but in one of two cases. First, if the people of America lay aside the Christian religion altogether, it may happen. Should this unfortunately take place, the people will choose such men as think as they do themselves. Another case is, if any persons of such descriptions should, notwithstanding their religion, acquire the confidence and esteem of the people of America by their good conduct and practice of virtue, they may be chosen. I leave it to gentlemens candor to judge what probability there is of the peoples choosing men of different sentiments from themselves (Elliot, 4:198-199, emp. added). http://www.apologeticspress.org
Thank you for posting those references to the Founders’ views on religion.