I am happy to find myself perfectly agreed with you, that we should begin by setting conscience free. When all men of all religions consistent with morals and property, shall enjoy equal liberty, property, or rather security of property, and an equal chance for honor and power, and when government shall be considered as having in it nothing more mysterious or divine than other arts or sciences, we may expect that improvements will be made in the human character and the state of society.

John Adams (1735–1826) American lawyer, Founding Father, statesman, US President (1797–1801)
Letter (1785-04-08) to Dr. Price
    (Source)

This quote is almost always given in the following, paraphrased form:

We should begin by setting conscience free. When all men of all religions shall enjoy equal liberty, property, and an equal chance for honors and power we may expect that improvements will be made in the human character and the state of society.

This simplifies his statement for religious tolerance (indeed, full social integration of all religions "consistent with morals and property"), but omits his stance (which he speaks to in the rest of the letter) on government properly being a secular organization, rather than sovereign rulers being being imbued with divine right from God.