The Constitution bars the federal government from favoring the Catholic religion. Article VI of the constitution says: “No religious test shall ever be required as a qualification to any office or public trust under the United States.” This clause prevents the federal government from requiring a profession of belief in Christ and the Catholic Church by office holders. Furthermore the establishment clause of the First Amendment states that “Congress shall make no law respecting an establishment of religion.” The dominant view interprets this clause to mean that the government may not “pass laws which aid one religion, aid all religions, or prefer one religion over another.” 
While it is true that there is another interpretation of the establishment clause called nonpreferentialism, which is more accommodating to religion, this interpretation also does not conform with Catholic doctrine. According to the nonpreferentialist view the establishment clause of the First Amendment was intended only to prevent “the establishment of a national church or religion, or the giving of any religious sect or denomination a preferred status.”  In other words the government can support religion in general but it cannot prefer any one religion over another. But this view has been condemned by Blessed Pope Pius IX in his Encyclical Quanta Cura. In it Pius IX condemned the view that “human society should be constituted and governed…. without making any distinction between the true religion and false religions.”  Pius IX also wrote that this was an “evil opinion” and must be “absolutely held as reprobated, denounced and condemned by all the children of the Catholic Church.”  This condemned error attacks “that salutary influence which the Catholic Church, by the institution and command of her Divine Author, ought to exercise even to the consummation of the world, not only over individual men, but nations, peoples, and sovereigns.”  And Leo XIII affirmed that “it is not lawful for the State, any more than for the individual,… to hold in equal favour different kinds of religion.” 
It is obvious which religion the Popes referred to as the “true religion” which the state should “favour” and that is the Catholic religion. Thus rulers have an obligation not only to favor religion but to favor one and only one religion: the Roman Catholic religion.
 Everson v. Board of Education, 330 U.S. 1, 16 (1947).
 Cord, Separation of Church and State: Historical Fact and Current Fiction (1982).
 Pius IX, Encyclical Quanta Cura (1864) 3.
 Quanta Cura 6.
 Leo XIII, Encyclical Immortale Dei (1885) 35.