“Government in our democracy, state and national, must be neutral in matters of religious theory, doctrine, and practice. It may not be hostile to any religion or to the advocacy of no-religion; and it may not aid, foster, or promote one religion or religious theory against another or even against the militant opposite. The First Amendment mandates governmental neutrality between religion and religion, and between religion and non-religion.”
  "As early as 1872, this Court said: ‘The law knows no heresy, and is committed to the support of no dogma, the establishment of no sect.’ Watson v. Jones, 13 Wall. 679, 728. This has been the interpretation of the great First Amendment which this Court has applied in the many and subtle problems which the ferment of our national life has presented for decision within the Amendment’s broad command."
–Justice Fortas, Epperson v. Arkansas, 393 U.S. 97, 103-104 (1968)

“The [First] Amendment’s purpose was not to strike merely at the official establishment of a single sect, creed or religion, outlawing only a formal relation such as had prevailed in England and some of the colonies. Necessarily it was to uproot all such relationships. But the object was broader than separating church and state in this narrow sense. It was to create a complete and permanent separation of the spheres of religious activity and civil authority by comprehensively forbidding every form of public aid or support for religion.”
–Justice Rutledge, Everson v. Board of Education, 330 U.S. 1, 31-32 (1947)

“There cannot be the slightest doubt that the First Amendment reflects the philosophy that Church and State should be separated. And so far as interference with the `free exercise’ of religion and an `establishment’ of religion are concerned, the separation must be complete and unequivocal. The First Amendment within the scope of its coverage permits no exception; the prohibition is absolute. The First Amendment, however, does not say that in every and all respects there shall be a separation of Church and State. Rather, it studiously defines the manner, the specific ways, in which there shall be no concert or union or dependency one on the other. That is the common sense of the matter.”
— Justice Douglas, Zorach v. Clauson, 343 U.S. 306 (1952)

“We repeat and again reaffirm that neither a State nor the Federal Government can constitutionally force a person `to profess a belief or disbelief in any religion.’ Neither can constitutionally pass laws or impose requirements which aid all religions as against non-believers, and neither can aid those religions based on a belief in the existence of God as against those religions founded on different beliefs.”
–Justice Black, Torcaso v. Watkins, 367 U.S. 488 (1961)

“The Constitution mandates that the government remain secular, rather than affiliating itself with religious beliefs or institutions, precisely in order to avoid discriminating against citizens on the basis of their religious faiths. Thus, the claim that prohibiting government from celebrating Christmas as a religious holiday discriminates against Christians in favor of nonadherents must fail, since it contradicts the fundamental premise of the Establishment Clause itself. In contrast, confining the government’s own Christmas celebration to the holiday’s secular aspects does not favor the religious beliefs of non-Christians over those of Christians, but simply permits the government to acknowledge the holiday without expressing an impermissible allegiance to Christian beliefs.”
— Justice Blackmun, Allegheny County v. Greater Pittsburgh ACLU, 492 U.S. 573 (1989)

“The wholesome ‘neutrality’ of which this Court’s cases speak thus stems from a recognition of the teachings of history that powerful sects or groups might bring about a fusion of governmental and religious functions or a concert or dependency of one upon the other to the end that official support of the State or Federal Government would be placed behind the tenets of one or of all orthodoxies. This the Establishment Clause prohibits. And a further reason for neutrality is found in the Free Exercise Clause, which recognizes the value of religious training, teaching and observance and, more particularly, the right of every person to freely choose his own course with reference thereto, free of any compulsion from the state. This the Free Exercise Clause guarantees. Thus, as we have seen, the two clauses may overlap. As we have indicated, the Establishment Clause has been directly considered by this Court eight times in the past score of years and, with only one Justice dissenting on the point, it has consistently held that the clause withdrew all legislative power respecting religious belief or the expression thereof. The test may be stated as follows: what are the purpose and the primary effect of the enactment? If either is the advancement or inhibition of religion then the enactment exceeds the scope of legislative power as circumscribed by the Constitution. That is to say that to withstand the strictures of the Establishment Clause there must be a secular legislative purpose and a primary effect that neither advances nor inhibits religion. The Free Exercise Clause, likewise considered many times here, withdraws from legislative power, state and federal, the exertion of any restraint on the free exercise of religion. Its purpose is to secure religious liberty in the individual by prohibiting any invasions thereof by civil authority. Hence it is necessary in a free exercise case for one to show the coercive effect of the enactment as it operates against him in the practice of his religion. The distinction between the two clauses is apparent – a violation of the Free Exercise Clause is predicated on coercion while the Establishment Clause violation need not be so attended.”
–Justice Clark, Abington School Dist.. v. Schempp, 374 U.S. 203 (1963)

“The place of religion in our society is an exalted one, achieved through a long tradition of reliance on the home, the church and the inviolable citadel of the individual heart and mind. We have come to recognize through bitter experience that it is not within the power of government to invade that citadel, whether its purpose or effect be to aid or oppose, to advance or retard. In the relationship between man and religion, the State is firmly committed to a position of neutrality. Though the application of that rule requires interpretation of a delicate sort, the rule itself is clearly and concisely stated in the words of the First Amendment. ”
–Justice Clark, Abington School Dist.. v. Schempp, 374 U.S. 203 (1963)

“While the Free Exercise Clause of the First Amendment is written in terms of what the State may not require of the individual, the Establishment Clause, serving the same goal of individual religious freedom, is written in different terms.”
“Establishment of a religion can be achieved in several ways. The church and state can be one; the church may control the state or the state may control the church; or the relationship may take one of several possible forms of a working arrangement between the two bodies. Under all of these arrangements the church typically has a place in the state’s budget, and church law usually governs such matters as baptism, marriage, divorce and separation, at least for its members and sometimes for the entire body politic.”
–Justice Douglass, concurring, Abington School Dist.. v. Schempp, 374 U.S. 203 (1963)

“While our institutions reflect a firm conviction that we are a religious people, those institutions by solemn constitutional injunction may not officially involve religion in such a way as to prefer, discriminate against, or oppress, a particular sect or religion. Equally the Constitution enjoins those involvements of religious with secular institutions which (a) serve the essentially religious activities of religious institutions; (b) employ the organs of government for essentially religious purposes; or (c) use essentially religious means to serve governmental ends where secular means would suffice. The constitutional mandate expresses a deliberate and considered judgment that such matters are to be left to the conscience of the citizen, and declares as a basic postulate of the relation between the citizen and his government that ‘the rights of conscience are, in their nature, of peculiar delicacy, and will little bear the gentlest touch of governmental hand.’"
–Justice Brennan, concurring, Abington School Dist. v. Schempp, 374 U.S. 203 (1963)

“The ‘establishment of religion’ clause of the First Amendment means at least this: Neither a state nor the Federal Government can set up a church. Neither can pass laws which aid one religion, aid all religions, or prefer one religion over another. Neither can force nor influence a person to go to or to remain away from church against his will or force him to profess a belief or disbelief in any religion. No person can be punished for entertaining or professing religious beliefs or disbeliefs, for church attendance or non-attendance. No tax in any amount, large or small, can be levied to support any religious activities or institutions, whatever they may be called, or whatever from they may adopt to teach or practice religion. Neither a state nor the Federal Government can, openly or secretly, participate in the affairs of any religious organizations or groups and vice versa.”
–Justice Black, Everson v. Board Of Education Of Ewing TP., 330 U.S. 1 (1947)

“We are all agreed that the First and Fourteenth Amendments have a secular reach far more penetrating in the conduct of Government than merely to forbid an `established church.’. . . We renew our conviction that `we have staked the very existence of our country on the faith that complete separation between the state and religion is best for the state and best for religion.'”
–Justice Frankfurter, concurring, Everson v. Board Of Education Of Ewing TP., 330 U.S. 1 (1947)

“Now as when it was adopted the price of religious freedom is double. It is that the church and religion shall live both within and upon that freedom. There cannot be freedom of religion, safeguarded by the state, and intervention by the church or its agencies in the state’s domain or dependency on its largesse. The great condition of religious liberty is that it be maintained free from sustenance, as also from other interferences, by the state. For when it comes to rest upon that secular foundation it vanishes with the resting. Public money devoted to payment of religious costs, educational or other, brings the quest for more. It brings too the struggle of sect against sect for the larger share or for any. Here one by numbers alone will benefit most, there another. That is precisely the history of societies which have had an established religion and dissident groups. It is the very thing Jefferson and Madison experienced and sought to guard against, whether in its blunt or in its more screened forms. The end of such strife cannot be other than to destroy the cherished liberty. The dominating group will achieve the dominant benefit; or all will embroil the state in their dissensions.”
–Justice Rutledge, (quoting James Madison’s “Remonstrance”), Everson v. Board Of Education Of Ewing TP., 330 U.S. 1 (1947)

Although the case did not deal with Church/State issues, this quote is impeccable –
"If there is any fixed star in our constitutional constellation, it is that no official, high or petty, can prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion or force citizens to confess by word or act their faith therein."
–Justice Jackson, West Virginia State Board Of Education v. Barnette, 319 U.S. 624 (1943)