The Founders, bless their hearts, did not always live up to the Constitution that they wrote. For example, the Alien and Sedition Acts were clearly unconstitutional under the 1st and 5th amendments but were nonetheless signed into law by John Adams. To give a closer example, Massachusetts had an established church (Congregationalist) until 1831, supported directly by the taxpayers. The deeds of the Founders do not make the basis for our laws, the Constitution does.
Christianity was overwhelmingly the religion of Americans, still is. No founder ever decried the fact that there was a dominant religion.
They certainly decried the presence of dominant sects. Then, as now, you cannot attend a "Christian" church. Every church is Anglican, Baptist, Congregationalist, Quaker, etc. Also,a key purpose of the Bill of Rights is protceting the rights of those who hold minority or unpopular beliefs. The fact that a majority of a community believe in one particular sect does not give them the right to tax everyone support it.
Getting back to the Everson, do you think government can prefer one religion to another in passing laws and policies? If a city council donates land for a Catholic Churce while turning down zoning for a Synagogue is that in accord with the Constitution? I'd say it is clearly unconstitutional. So would James Madison. Here is a veto message from his presidency:
February 28, 1811So even in the early days of the republic, politicians were trying to curry favor by supporting the locally popular religion and even then wiser minds realized that such practices were contrary to the Establishment Clause. Whether an acre or a square foot, the principle is the same.
To the House of Representatives of the United States:
Having examined and considered the bill entitled "An act for the relief of Richard Tervin, William Coleman, Edwin Lewis, Samuel Mims, Joseph Wilson, and the Baptist Church at Salem Meeting House, in the Mississippi Territory," I now return the same to the House of Representatives, in which it originated, with the following objection:Because the bill in reserving a certain parcel of land of the United States for the use of said Baptist Church comprises a principle and precedent for the appropriation of funds of the United States for the use and support of religious societies, contrary to the article of the Constitution which declares that "Congress shall make no law respecting a religious establishment."
Founders are on record supporting the interpretive use of historical knowledge of laws and practices.
I agree not to use the Alien and Sedition Acts as an example since they were later declared unconstitutional.
Your example of the Massachusetts established Congregationalist church is out of order, since the First Amendment did not then apply to Massachusetts law.