Numerous passages in the context of Kant’s philosophy of religion show without doubt his acquaintance with Lessing. But apart from the obvious affinity and agreement between Kant and Lessing with regard to many substantial questions, serious differences cannot be overlooked; the frequently diagnosed closeness and widely suspected “harmony” between the two is probably also the primary reason why important factual differences and controversial aspects have so far usually been neglected or ignored in research, although they still continue to raise problems and controversies in the context of the Enlightenment and the philosophy of religion. Although Lessing and Kant are both committed to the ideas of the Enlightenment and also appear as “related in essence”, above all with regard to religio-philosophical questions, Kant’s “moral determined monotheism” also contains an obvious criticism of Lessing’s religio-philosophical doctrines. This is also obvious in Kant’s — direct and indirect — confrontation with the “Ring Parable” in Lessing’s drama Nathan the Wise. The criticism that becomes apparent there concerns above all the question of “principles” left unclarified by Lessing, the “equal rank” of the monotheistic religions which he claimed, and the asserted “competition of religions.” I investigate some of the main points of this criticism.