Sidonius, Michael, a prelate of the Church of Rome who became noteworthy through his participation in many of the most important transactions connected with the Reformation, but whose family name was Helding, was born in Baden in 1506, studied at Tubingen, and entered the priesthood at Mayence, where he became cathedral preacher and rector of the cathedral school in 1531. In 1538 he was made suffragan to the archbishop of Mayence, and received from pope Paul III the. title of bishop of Sidon in partibus infidelium, which gave him the name of Sidonius, by which he is commonly known. The Theological Faculty of Mayence conferred the degree of D.D. on him in 1543, and afterwards he for a time represented the elector of Mayence in the Council of Trent. In 1547 he was made imperial councillor by Charles V. He took possession of the pulpit of the reformer Musculus during the Diet of Augsburg, and from it preached a series of anti-Lutheran sermons (Sleidani de Statu Rel. etc. [Frankf. 1786]). In 1548 he served with Jul. v. Pflug,. bishop of Naumburg, and with Joh. Agricola, the court preacher at Eisleben, on the commission which drew up the Augsburg Interim (q.v.), after Which he was sent by his archbishop to promote the execution of the Interim at Frankfort. Prince George of Anhalt was at this time coadjutor of the bishopric of Merseburg, having been chosen by a majority of the canons; but the emperor declared against him and nominated Sidonius in his stead (Seckendorf, Comment. de Lutheranismo [Lips. 1694], lib. iii, c. 30, § 117, p. 497 sq.), though the opposition raised against the measure delayed his investiture until 1550. In that year Sidonius was present at the Diet of Augsburg, in 1556 at that of Regensburg, in 1557 at the Colloquy of Worms, where he contributed according to his ability to render reconciliation impossible by his addresses, and by introducing at the sixth session a rejoinder to a declaration of facts submitted by the Protestants, in which he not only defended the traditional teachings and practice of Rome, but also asserted that the interpretation of difficult and controverted passages of Scripture belongs rightfully to the Romish Church. The Romish collocutors finally refused to continue the negotiations (Salig, Vollst. Hist. d. Augsb. Consf. [Halle, 1735], iii, 292 sq.), To the. honors already enjoyed by Sidonius was added by the emperor in 1558 the office of judge in chambers. He died Sept. 30, 1561 , at Vienna, and was buried in St. Stephen's Church. His writings include a Catechismus Mogunt. s. Institut. ad Christ. Pietatem (frequently reprinted and much controverted by Protestants):-the Sleidani de Statu Rel. already mentioned:-Decreta Concil. General. Moguntini:-Instructio Visitatorum, and Explicatio Paraphrast. Missce. See Unschuld. Nachrichten, 1715, p. 394 sq.; 1716, p. 7 sq.