He seems to describe a phenomenon of sectarian diversity within Christianity leading to an evolution towards Deism. A shorter way of describing it: Christianity rationalizes towards Deism.
Another consideration still more powerful, accelerated the progress of moral improvement, and constantly diminished the force of attachment toward the Christian system. Every new sect discarded some of the absurdities of that from which it had separated, and passed a general sentiment of condemnation upon all those who were in the rear of this long and religious train. Luther and Calvin hurled their religious thunderbolts against the power and absurd tenets of the Church of Rome, and especially against the Pope, by whom this Church was governed. The Armenians, the Arians, the Socinians, and the Universalists, successively followed, with a purifying hand of reason, pruning and lopping off the decayed branches of the old theological tree, approaching still nearer to the source and principles of nature, till at length, by regular progression, the human mind discovered, that moral principle was placed upon a more solid foundation than the reveries of sectarian fanaticism. It has been in this manner that some portion of society has once more obtained a true idea of the religion of nature, or of that which may be denominated pure and simple Deism.
It is this religion which, at the present period of the world, creates, such frightful apprehensions in the household of faith, and threatens to shake to the centre the chief corner stone on which the Church is built. These apprehensions are daily disclosed by Christian professors, and they depict in such strong colours the fatal effects of Deism, that ignorant fanaticism believes it to be an immoral monster, stalking with gigantic strides over the whole civilized world, for the detestable purpose of producing universal disorder, and subverting all the sound principles of social and intelligent existence. Such are the horrid ideas which the enemies of this pure and holy religion are every where propagating amongst their credulous and deluded followers. This circumstance renders it necessary, that the true idea of Deism be fairly stated, that it may be clearly understood by those whose minds have hitherto been darkened by the mysteries of faith.
Deism declares to intelligent man the existence of one perfect God, Creator and Preserver of the Universe ; that the laws by which he governs the world are like himself immutable, and, of course, that violations of these laws, or miraculous interference in the movements of nature, must be necessarily excluded from the grand system of universal existence; that the Creator is justly entitled to the adoration of every intellectual agent throughout the regions of infinite space ; and that he alone is entitled to it, having no co-partners who have a right to share with him the homage of the intelligent world. Deism also declares, - that the practice of a pure, natural, and uncorrupted virtue, is the essential duty, and constitutes the highest dignity of man; that the powers of man are competent to all the great purposes of human existence; that science, virtue, and happiness, are the great objects which ought to awake the mental energies, and draw forth the moral affections of the human race.
I'm of the mind that Elihu Palmer, Thomas Paine, and Ethan Allen are the notable "Deists" of the American Founding; but none of them was a "key Founder." Likewise there were orthodox Christians among the Founders; but the "key Founders" (the first four Presidents, Ben Franklin and a few others) were neither strict Deists nor orthodox Christians, but something in the middle.