Conrad Vorstius (German: Konrad von der Vorst; Latin: Conradus Vorstius; 19 July 1569 – 29 September 1622) was a German-Dutch controversial Remonstrant theologian, successor to Jacobus Arminius in the theology chair at Leiden University, and—as a theologian—second to Johannes Uytenbogaert in the Remonstrant Society. His appointment, and the controversy surrounding it, became an international matter in the political and religious affairs of the United Provinces during the Twelve Years' Truce, supplying a pretext for the irregular intervention of King James I of England in those affairs. Vorstius published theological views which were taken by some to show sympathy with the Socinians, and was declared unworthy of his office by the Calvinists at the Synod of Dort in 1619.

Vorstius was born one of ten children at Cologne on 19 July 1569. His parents Theodor Vorstius and his wife Sophia Starckia were Roman Catholic and wanted him to become a Catholic priest, but the parents converted to Protestant belief before he could undertake these studies. He received the rudiments of his education at Bedburdyck (Jüchen, Germany) for five years, before studying at Düsseldorf from 1583 to 1587, and also at Aix-la-Chapelle. He entered the college of St. Lawrence in Cologne, where he should have taken his Bachelor's and master's degrees, but was unable in conscience to take the required oath of obedience to the decrees of the Council of Trent.

His parents not having much money, he went into practical affairs as a Purchaser for two years, where he learnt to serve the business and acquired skills in reckoning and in French and Italian. In 1589 he took up his studies once more and entered the Herborn Academy from 1589 until 1593, where he devoted himself fully to Theology under Johannes Piscator. He had not neglected his Philosophical studies, however, having often taken part in theological and philosophical disputations there. In 1590-1591 he began to take private pupils, instructing the sons of dignitaries who afterwards held him in friendship. He proceeded to Heidelberg on 12 April 1593, focusing on theology on 12 April 1594, and he was publicly created and declared a Doctor of Theology (SS. Theologiae Doctor) on 4 July 1594.

In December 1595 he travelled with two companions to Basel and Geneva, where he attended lectures by Theodore Beza, and earned a considerable reputation for himself. His disputations De Sacramentis (Basel, 1595) and De Causis Salutis (1595) won him the offer of a position as teacher for 120 crowns a year, with the approval of Beza and of Johann Jakob Grynaeus. Vorstius, however, decided to return to his own country, and went instead to Burgsteinfurt in 1596, in the County of Bentheim where, thanks to a recommendation from Beza and David Pareus, he taught at Graf von Bentheim's Hohe Schule for fifteen years. In Burgsteinfurt Vorstius defended the Reformed religion against the Catholic theologian Robert Bellarmine. He also received offers of teaching positions at Saumur and Marburg, but was unable or unwilling to leave the service of the Bentheims.

At about this time, by 1597, Vorstius married and embarked upon fatherhood. It was in Burgsteinfurt that his publications De Praedestinatione (Burgsteinfurt, 1597), De Sancta Trinitate (1597), and De Persona et Officio Christi (1597) brought him under suspicion of Socinianism: his patron advised him to clear himself of the charge, and in 1599 he travelled to Heidelberg for that purpose and successfully defended his orthodoxy before the theological faculty there. After this he was fully reinstated and advanced in Burgsteinfurt, in 1605 receiving the additional appointments of preacher, and Consistorial Assessor.

In the context of the commencement of the Twelve Years' Truce in 1609, Vorstius published a treatise against Cardinal Bellarmine in 1610. Following the death of Arminius, which created a vacancy in the Theology chair at Leiden, in 1610 Vorstius accepted a calling to succeed him. He was "praised enthusiastically by indisputably orthodox divines at Heidelberg and Arnhem as worthy of the post". He was nominated for the chair by moderate members of the Remonstrant party who approved of his support for public freedom of opinion, "having defended the toleration of diverse opinions in his book against Bellarmine." It was hoped he would also be acceptable to some of the Contra-Remonstrants, on account of his orthodox background. His acceptance of the appointment, however, gave offence to the Count of Bentheim, and he made an Apology (a Declaration) to the university concerning his beliefs and practises.

Vorstius was a very troubling kind of academic, who could challenge fundamental tenets of scholastic theology. He presented such arguments without endorsing them as points of belief, for example that the divine essence, if (considered as a body, in the broadest meaning of that term) it had extent and magnitude, could not also be infinite. Similarly (regarding Predestination), whereas future outcomes were conditional upon elective actions in the present, the Deity given to managing human affairs must not also have full fore-knowledge of them: hence the divine will, though essential in itself, in its contingent or arbitrary operations might be mutable, and not uniform in its motions. In 1610 he reprinted his Tractatus Theologicus de Deo, sive de Natura et Attributis Dei: Decem Disputationes, which had first seen the light in 1606, and was dedicated to Maurice, Landgrave of Hesse-Kassel.

The appointment of Vorstius (a prerogative of the magistrates) gave the opponents of Arminius the opportunity to make a political intervention in the name of the defence of the Christian religion. His teaching appeared heterodox and deeply sceptical, seeming to stray from Christianity, even from Theism altogether. His statements in the Tractatus led the Counter-Remonstrants to accuse him of sympathy for, and encouragement of, the loathed Socinian heterodoxy, a system questioning the Triune and eternal nature of God. In 1611 therefore he gave answer in his Epitome Exegeseos Apologeticae.