He had been teaching for eight years in Parc, the Premonstratensian house near Louvain, when he was appointed professor of theology at the university. Like Michael Baius, who was his senior colleague, Hessels preferred drawing his theology from the Church Fathers, especially from Augustine of Hippo, rather than from the Schoolmen.
In 1559 he accompanied the elder Cornelius Jansenius (later Bishop of Ghent) and Baius to the Council of Trent and took an active part. He prepared the decree "De invocatione et reliquiis sanctorum et sacris imaginibus".
Even at Trent the Scholastic party found fault with his departure from the beaten tracks of learning; after his return the attacks continued. Hessels, however, used his energy against the Protestants. He was an opponent of Georgius Cassander.
He upheld the doctrine of the Immaculate Conception (impugned by Baius), and he is a protagonist of papal infallibility in his "De perpetuitate Cathedræ Petri et ejus indefectibilitate", which is an appendix to his polemical work "Confutatio novitiae fidei quam specialem vocant, adv. Johannem Monhemium" (Louvain, 1565).
His other polemical works are:
- "De invocatione sanctorum . . . censura" (1568);
- "Probatio corporalis præsentiæ corporis et sanguinis dominici in Eucharistia (Cologne, 1563);
- "Confutatio confessionis hæreticæ, teutonice emissæ, qua ostenditur Christum esse sacrificium propitiatorium" (Louvain, 1565);
- "Oratio de officio pii viri exsurgente et vigente hæresi" (Louvain, 1565);
- "Declaratio quod sumptio Eucharistiæ sub unica panis specie neque Christi præcepto aut institutioni adversetur" (Louvain).
He also wrote commentaries:
- "De Passione Domini" (Louvain, 1568);
- "de I Tim. et I Petri" (Louvain, 1568);
- "Com. de Evang. Matthæi" (Louvain, 1572); "Com. de Epp. Johannis" (Douai, 1601).
His chief dogmatic work is a Catechism, first published in 1571, by Henry Gravius, who removed from it all traces of Baianism.
- Mathijs Lamberigts, Leo Kenis (1994), L'Augustinisme à l'ancienne Faculté de théologie de Louvain, pp. 99–122
- Johannes, Joannes, Jan, John Hessels or Hessel, Hesselius or Hasselius.
- Steven Vanden Broecke, The Limits of Influence: Pico, Louvain, and the Crisis of Renaissance Astrology (2003), p. 224.
- Rob van der Schoor, The Reception of Cassander in the Republic in the Seventeenth Century , p. 101 in Christiane Berkvens-Stevelinck, Jonathan Irvine Israel, Guillaume Henri Marie Posthumus Meyjes (editors), The Emergence of Tolerance in the Dutch Republic (1997).