Until I read the interview with Justice Thomas, to which Joe links below, I had not read the Judiciary Act of 1789 in quite some time, and had forgotten that it prescribes an oath of office for Justices (Section 8).
Justice Thomas notes that he takes his oath of office seriously. The text of the oath reads: To "solemnly swear or affirm, that I will administer justice without respect to persons, and do equal right to the poor and to the rich, and that I will faithfully and impartially discharge and perform all the duties incumbent on me as , according to the best of my abilities and understanding, agreeably to the constitution, and laws of the United States. So help me God."
What stands out is the line, "and do equal right to the poor and to the rich."
Given the time and place at which the oath was written, the language was probably ultimately traceable to Leviticus 19:15: "You shall do no injustice in judgment; you shall not be partial to the poor or defer to the great, but in righteousness shall you judge your neighbor." If so, it might have interesting implications for our establishment clause jurisprudence.
And the "So help me God" part of the oath suggests that they members of the First Congress agreed with John Locke’s Letter Concerning Toleration that atheists could not be good citizens, for "those are not at all to be tolerated who deny the being of a God. Promises, covenants, and oaths, which are the bonds of human society, can have no hold upon an atheist."