Ursinus was soon afterwards called to the Collegium Sapientiae in Heidelberg, and to its duties afterwards added the chair of dogmatics. He began his theological prelections Sept. 1, 1562, and in the following year undertook also the delivery of the Sunday-afternoon sermon on the catechism. To these various duties he added the formation of a constitution for the churches of the Palatinate, in which he was aided by Olevian, but whose defense devolved on him alone. It was in the prosecution of this work that he began his active literary life. He wrote a Verantwortung against criticisms and perversions of the Heidelberg Catechism, which formed the principal element in the new constitution, Antwort auf etlicher Theologen Censur, and other works. When the Maulbronn Convention grew out of these discussions, SEE MAULBRONN. Ursinus was one of the collocutors for the Palatinate, and demonstrated by his readiness and keenness that he was one of the ablest disputants of the time. The Wirtembergers having violated the agreement to refrain from publishing the proceedings, the Heidelbergers were obliged to respond; and the duty of correcting the perversions which had gone out before the public devolved again on Ursinus. From this time onward he was involved in the controversy about the correct interpretation of Art. 10 of the Augsburg Confession, in which the strict Lutherans insisted that Luther's writings, especially his polemical writings, should be considered the only guide, and endeavored to deprive all who did not hold their view of the legal standing assured to those who accepted the Confession as a statement of their faith. Weary of the endless dispute, Ursinus closed his share in the controversy in 1566, with the determination to write no more. He was worn out. His health was impaired, and he was obliged to seek relief from excessive labors by resigning the chair of dogmatics to Hieronymus Zanchius, Feb. 10, 1568. A few months later, however, a new conflict demanded his attention. George Withers, an Englishman, had defended in a disputation at Heidelberg the thesis that the administration of ecclesiastical discipline in all its extent belongs properly to the ecclesiastical ministerium in connection with an organized presbyterate; and Qlevian had endorsed that opinion, while Erastus opposed it. Each side gained adherents without being able to intimidate its opponents. Beza and Bullinger were called on for advice, and, eventually, Ursinus was required by the elector to state his views. He did this in 1569, in so candid and kindly a manner as to win approval even from those who did not accept his conclusions. The elector finally decreed the erection of presbyteries and the execution of discipline.
The accession of the elector Louis inaugurated a new order of things in the Palatinate, under which Lutheranism was able to regain its predominance. The Collegium Sapientic was closed in September, 1577, and Ursinus was dismissed from his post. A professorship in Lausanne was at once offered him, but he declined it, and accepted, instead, a call to Neustadt, where the theology of the Reformed Church found a refuge in the Collegium Illustre Casimirianum. He had previously published, in Latin and German, the confession of faith appended to the late elector's will. (1577), and was soon afterwards commissioned, in connection with Zanchius, to draw up for the Frankfort Synod (September, 1578) a confession which should be accepted in the Reformed churches of all European countries. This office he declined on the grounds of ill-health and distrust of his ability. He began his lectures on Isaiah May 26,1578, and subsequently participated in the conflict over the acceptance of the Formula Concordice, having contributed the most powerful argument in opposition to that measure. He died March 6, 1583, at Neustadt. His literary remains were entrusted to Prof. Jungnitz, and he, with other friends of the departed scholar, collected and published many works which, until then, existed only in MS., and gave the author's name to others which had previously, been anonymously published. The Heidelberg Catechism, with notes, and Lectures on the Organon of Aristotle, etc., were published at Neustadt. Pareus, at a later day, issued a corrected edition of the Exposition of the Catechism (Brem., 1623, 8vo); and a complete edition of Ursinus's works was issued by Reuter, his pupil and immediate successor in the Sapientiae.
See Adam, Vit. German. Theologorum; Heppe, Gesch. d. deutsch. Protestantismus; id. Dogmatik d. deutsch. Protestantismus, 1, 158-160; Sudhoff, Olevianus u. Ursinus (Culberfeld, 1857); id. Leben d. Vater d. reformirt. Kirche, vol 8; Gillet, Cratos von Crafftheim (Frankf. 1860); Herzog Real-Encyklop. s.v.; Smith's Hagenbach, Hist of Doctrinses, 2, § 222, and § 223 a, 4.