Americans United for Separation of Church and State
Founded in 1947, Americans United for Separation of Church and State — commonly called Americans United (AU) for short — is a nonpartisan organization "dedicated to preserving the constitutional principle of church-state separation as the only way to ensure religious freedom for all Americans." It interprets the Establishment Clause of the First Amendment of the U.S. Constitution to mean that government must stay completely separate from religion. Barry C. Lynn is the Executive Director.
The First Amendment states “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof….” In the view of AU, this forbids more than the establishment of a national religion; it also forbids laws or actions respecting an establishment of religion. It recommends the "endorsement test" stated by Justice Sandra Day O'Connor; that government endorsement of religion sends a
message to non-adherents that they are outsiders, not full members of the political community, [and the] accompanying message to adherents that they are insiders, favored members of the political community.”
As President James Madison wrote “The Constitution of the U.S. forbids everything like an establishment of a national religion.” It also cites, President Thomas Jefferson's comment“I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should make no law respecting an establishment of religion, or prohibit the free exercise thereof, thus building a wall of separation between church and state.” AU, therefore, rejects claims that the United States is in any way a Christian nation or should be an Islamic state.
AU does not endorse candidates for political office, as consistent with its 501(c)(3) status. It does say
We do oppose efforts by the Religious Right to impose its theological views on the public by governmental action. The Religious Right's attempt to force all Americans to accept its religious doctrines as law is one of the greatest threats to religious freedom today.
It does, however, actively participate in court cases where it believes the wall of separation is being breached.
Regarding faith-based initiatives, its policy is "Churches and other houses of worship are free to spread their beliefs. But they are expected to raise their own funds through the voluntary contributions of the faithful. Taxation to support religion and religious endeavors was forbidden under the Founders’ constitutional framework. No American should be compelled through taxation to subsidize any religion." 
Pressure for such statements increased in 1996, when Sen. John Ashcroft (R-Missouri) introduced a “charitable choice” mechanism that would allow faith-based social service providers to be authorized to hire and fire based on religion, even in programs supported by Federal funds. President Bill Clinton opposed some sections, and blocked others with signing statements.
Under President George W. Bush, however, faith-based initiatives became a major thrust of his Administration.
One program that it opposed was the InnerChange Freedom Initiative in the Iowa state prisons, as operated by Prison Fellowship Ministries. AU was joined by the Anti-Defamation League and American Jewish Committee, initially in the Iowa state courts in February 2003, and eventually in the United States Court of Appeals for the Eighth Circuit; part of the challenge in Americans United for Separation of Church & State v. Prison Fellowship Ministries (also known as Ashburn v. Mapes) succeeded and part was reversed. IFI chose not to appeal to the Supreme Court of the United States, and, in a settlement agreement, paid AU's attorney fees. Iowa terminated the IFI program in March 2008.
Public funds to religious schools
AU is concerned with pressure to provide tax funds to religious schools from both the Protestant Christian Right and from the Roman Catholic hierarchy. It quotes Christian Coalition founder as saying "the Constitution says nothing about the separation of church and state! They say vouchers would spell the end of public schools in America. To which we say, so what?" Groups including the Family Research Council, Focus on the Family, Concerned Women for America and others also support what they see as a breach of the wall in support of religion. 
It intervened blocking the Supreme Court challenge of Anderson v. Durham School Department, a Maine case in which parents of children in rural areas could obtain state funds to send their children to nonsectarian, but not religious, schools. 
- Frequently Asked Questions, Americans United
- The 'Faith-Based' Initiative: Churches, Social Services And Your Tax Dollars, Americans United
- Brief Amicus Curiae of Anti-Defamation League and American Jewish Committee on Behalf of Plaintiff-Appellees, supporting Affirmance, 22 November 2006
- Americans United for Separation of Church & State v. Prison Fellowship Ministriess/Ashburn v. Mapes, Americans United, 18 September 2009
- Should You Pay Taxes To Support Religious Schools?, Americans United
- "Catholics, Orthodox Jews seek tax subsidies for religious education", Church & State, 1 January 2007