[4] He served as a judge in the courts of the Roman Curia, and as ambassador of Pope Paul III to the German princes in negotiations for holding the Council of Trent.
On 17 August 1535, Pope Paul III issued a bull, Sublimis Deus, in which he appointed a commission of five cardinals (Piccolomini, Sanseverino, Ghinucci, Simonetti, and Cesi) and three bishops, one of whom was Van der Vorst, to carry out a reform of the city of Rome and the Roman Curia, with unlimited powers to uproot and punish all spiritual and secular transgressions, abuses, and errors.
[17] On 10 September 1536 Pierre van der Vorst was sent as Nuntius to Germany to announce to the various princes the opening of the ecumenical council, which was supposed to hold its first session at Mantua on 23 May 1537.
[20] The more difficult part of van der Vorst's assignment was to convince the various German princes, whether Catholic or Protestant, to support or at least cooperate with the summoning of the Council and its work.
[22] On 9 March Van der Vorst visited Halle, the residence of Cardinal Albert of Brandenburg, Archbishop of Mainz, with whom he engaged in discussions and where he also met Duke Henry of Brunswick-Wolfenbüttel.
Though he carried letters of introduction in addition to his mandate from the Pope, he was coldly received by the Dean of the Chapter of the Cathedral, and turned over to the consuls of the city instead for hospitality.
[25] On April 22 Hermann von Wied, the Archbishop of Cologne,[26] arrived in person, a rather tall old man in a full white beard, who escorted the Nuncio to his palace in Bonn.
[27] On 25 April, van der Vort held consultations with the Archbishop of Trier, Johann von Metzenhausen, who stated that he would by no means consent to the Pope's plans unless the other electors agreed, in which case he would follow the majority.
Paul III specifically addresses him, tibi qui locum unius ex causarum palatii Apostolici auditoribus de mandato nostro tenes, ac Prelatus domesticus noster.
He obtained a decree from Pope Paul III, dated 9 July 1543, granting him the privilege of filling all of the vacant benefices in his diocese during his visit, with a limit of the two months of August and September.
[30] Bishop Van der Vorst was present at the 9th and 10th sessions of the Council of Trent, which were held in Bologna, and he signed the decrees on 21 April and 1 June 1547.
[31] On 30 October 1548, Pope Paul III wrote a letter to the Emperor Charles V, commending Bishop van der Vorst, who was setting out for Flanders, for the sake of his health and to put his affairs there in order.