By "true religion" he means the Christian religion, aka biblical faith and morality.

"To promote true religion is the best and most effectual way of making a virtuous and regular people. Love to God and love to man is the substance of religion; when these prevail, civil laws will have little to do. ... The magistrate (or ruling part of any society) ought to encourage piety ... [and] make it an object of public esteem. Those who are vested with civil authority ought ... to promote religion and good morals among all their government."

John Witherspoon, United States Founding Father, Signer of the Declaration of Independence, "The Works of John Witherspoon, (Edinburgh: J. Ogle, 1815), Vol. IV, p. 265, "Sermon Delivered at Public Thanksgiving After Peace"