Michael Baius, Or De Bay , Michael (1513-1589), Belgian theologian, was born at Melun in Hainault in 1513. Educated at Louvain University, he studied philosophy and theology with distinguished success, and was rewarded by a series of academic appointments. In 1552 Charles V. appointed him professor of scriptural interpretation in the university. In 1563 he was nominated one of the Belgian representatives at the council of Trent, but arrived too late to take an important part in its deliberations. At Louvain, however, he obtained a great name as a leader in the anti-scholastic reaction of the 16th century. The champions of this reaction fought under the banner of St Augustine; and Baius' Augustinian predilections brought him into conflict with Rome on questions of grace, free-will and the like. In 1567 Pius V. condemned seventy-nine propositions from his writings in the Bull Ex omnibus afflictionibus. To this Baius submitted; though certain indiscreet utterances on the part of himself and his supporters led to a renewal of the condemnation in 1579 by Gregory XIII. Baius, however, was not disturbed in the tenure of his professorship, and even became chancellor of Louvain in 1575. He died, still in the enjoyment of these two dignities, in 1589. Baius is chiefly interesting as a forerunner of the more celebrated Cornelius Jansen (see Jansen). His writings are described by Harnack as a curious mixture of Catholic orthodoxy and unconscious tendencies to Protestantism; their most noticeable point is the great importance they attach to the fact of sin, both original and actual.

His principal works were published in a collected form at Cologne, 1696, 1 vol. 4to, in two parts; some large treatises have not been published. There is an excellent study of both books and author by Linsenmann, Michael Baius, und die Grundlegung des Jansenismus, published at Tübingen in 1867.