Who what exactly is this extremist animal that keeps being mentioned? And what exactly is this individual's ideology?
First, a separation of church and state extremist is one who wishes to divorce religion completely from government, in some cases to the point of absolute lunacy. Earlier this decade, for example, the American Atheists sued the city of Zion, IL because (gasp!) the city had a cross on its flag. They claimed that this violated the Constitutional principle of separation of church and state. In another instance, a federal judge in Shreveport, LA ruled that teaching abstinence in public schools was unconstitional for the same reason. But alas, this is what happens when we allow these people to operate unchecked.
Specifically, these extremists are known for their rabid opposition to teacher-led prayer in public schools. Not recognizing any compromise in this area, many of them are vocal in their opposition to a moment of silence in public schools. They are also known for their opposition to: nativity scenes on public parks, tax vouchers for those who wish to send their children to private schools instead of public schools (although they don't seem to have a problem with Pell Grants for college students who wish to attend Catholic Universities), displaying the Ten Commandments on Courtroom walls, prayer at public school graduations, religious symbols on public property, the teaching of creation in public schools, and host of other unquestionably disastrous practices.
There are several examples of Supreme Court cases which highlight the sad and dangerous legacy of the church/state separation demagogues as they attempt to redefine the scope of the First Amendment to support their extremist agenda:
1. Some separation of church and state extremists have tried to claim that New York City's program that sent public school teachers into parochial schools to provide remedial education to disadvantaged children pursuant to Title I of the Elementary and Secondary Education Act of 1965 constituted an "excessive entanglement" between church and state. See U.S. Supreme Court decisions Aguilar v. Felton (1985 - extremism affirmed) and Agostini v. Felton (1997 - extremism thankfully relieved). The ACLU and the Americans United for the Separation of Church and State submitted briefs of amici curiae urging the Supreme Court to rule against the disadvantaged children.
2. In 1985, the principal and superintendent of Westside High School (a secondary school in Omaha, Nebraska) cited the Establishment Clause as a reason for denying the request of Bridget Mergens to form a Christian club that would have the same privileges and meet on the same terms and conditions as other Westside student groups, except that it would have no faculty sponsor. Ms. Mergens took the case to court, and won at first. She lost on appeal at the 8th Circuit Court, and then later won in a 8-1 decision from the Supreme Court (Justice Stevens, ever the left-wing radical, the sole dissent). See Westside Community Bd. Of Ed. v. Mergens (1990). People for the American Way, that great bastion of liberal activism not living up to its name, filed a brief of amici curiae urging the Court to rule against Bridget Mergens.
3. As if barring Christian students from voluntarily organizing after hours on public school property and attempting to stifle remedial education to disadvantaged Catholic children was not enough, the separation of church and state extremists also attempted to prevent a deaf child's state-funded interpreter from accompanying him to his Roman Catholic school. The extremists argued that "the interpreter would act as a conduit for the child's religious inculcation, thereby promoting his religious development at government expense in violation of the Establishment Clause." The Supreme Court, however, when hearing this case, ruled on the side of sanity, allowing the deaf student to have a publicly-funded interpreter in class. See Zobrest v. Catalina Foothills Sch. Dist. (1993). The ACLU submitted a brief of amici curiae urging the Court to rule against the deaf student, as did Lee Boothby and Robert W. Nixon, who elsewhere represent the Americans United for the Separation of Church and State, but here represent the misnamed Council on Religious Freedom.
4. As if picking on disadvantaged children, Christian high school students, and deaf kids wasn't enough, the separation of church and state extremists also attacked the blind. Larry Witters, suffering from a progressive eye problem, was denied vocational rehabilitation assistance by the Washington Commission for the Blind, because, the state supreme court ruled, Larry Winters was attending a Christian college and the aid would "have the primary effect of advancing religion." On appeal, the United States Supreme Court ruled, without dissent, that Witters was entitled to the aid. See Witters v. Wash. Dept. of Services For Blind (1986). The ACLU and the Americans United for the Separation of Church and State submitted a brief of amici curiae urging the Court to rule against Mr. Witters.
5. In an effort that seems moderate when compared against the above quartet of extremist examples, the church/state mafia also tried to challenge a Minnesota law which allows state taxpayers to deduct expenses incurred in providing "tuition, textbooks and transportation" for their children attending an elementary or secondary school. They claimed that the law violated the First Amendment by providing financial assistance to sectarian institutions, because parents who sent their children to parochial schools can also claim the deduction. The High Court ruled, in a 5-4 decision, that the law was valid. The Americans United for the Separation of Church and State, rearing their ugly heads in failure again, filed a brief of amici curiae urging the Supreme Court to find the tax deduction unconstitutional. See Mueller v. Allen.
Now note: No one here is advocating the unification of church and state.
But somewhere between the complete joining of the ecclasiastical body with
the government and the total elimination of all things religious from the
public sector (including the type of nonsensical radicalism we see above),
there has got to be a happy medium. The key word here is moderation, which
deviates from the conventional wisdom and alienates the extremists, but
is sound in thought and practice.
 It is interesting to note that on the misnamed Constitutional
Principle web site, Mr. Allison, et. al. lists
a link to The People for the American
Way web site and calls the group an "organization with a track record
of defending religious liberty." Yet, as we have seen, The People for the
American Way urged the High Court to rule against the wishes of
a high school girl to form an after-hours Christian club within her public
school. If that's what Mr. Allison refers to as "defending religious liberty,"
it shows us where his brand of extremism is heading.