In the USA, the constitutional depiction of the relations between Church and State is contained in the First Amendment to the Federal Constitution which set up the 'establishment clause'. The establishment clause prohibits the Congress from passing a law that would establish an official church of the United States (i.e. to establish an official federal religion). The first attempt to give an extensive interpretation of the establishment clause was made in 1947 by Justice Black in Everson v. Board of Education of Ewing Township. The ratio decidendi of the Everson case raised the separation between Church and State to the status of a constitutional norm. In its opinion, the Court invoked Thomas Jefferson's metaphor of the wall of separation, used in a letter to the Danbury Baptist Association in 1802. As a consequence of a US Supreme Court's landmark decision in Everson, the phrase 'wall of separation' spread into the language of politics in America and, what is more important, was frequently referred to by the Supreme Court in its decisions concerning religious freedom in general, and the implementation of the establishment clause in particular. The first use of the above-mentioned phrase in the American political debate took place already in the colonial period. The metaphor of the wall of separation was used by Roger Williams in 1644 in his political pamphlet containing arguments against the institution of a state church in Massachusetts Bay Colony in New England. The article is aimed at reconstruction of doctrinal origins of the metaphor of the wall of separation and its role in shaping perception of proper relations between Church and State in the United States.