The author offers first (Part I) an analysis of the origins of the conception of Catholic tolerance in the work of Thomas Aquinas. He argues here that Aquina’s doctrine is grounded on the double dichotomy of "believers/non-believers" and "truth/error" from which the idea of tolerance flows as acceptance of a lesser evil in order to avoid a greater one. He also points out (Part II) that for a long period, this idea was reinforced by the adoption by confessional governments (Catholic and reformed) of a policy of coercion against non-believers in the official religion. In part III the author characterizes positive tolerance, by reference to John Locke’s doctrine. He concludes, in light of the definitions of the Vatican II Council, that liberal tolerance and Catholic tolerance are compatible.