[1] During the following years, he traveled in various European countries, spending some time at the German College in Rome; in 1728, he was ordained a priest, and, formally admitted to the chapter of St. Simeon in 1732, he became a professor of the Pandects at the University of Trier.
In that capacity, he had plenty of opportunities to study the effect of the influence of the Roman Curia on the internal affairs of the Empire, notably in the negotiations that preceded the elections of the emperors Charles VII and Francis I, in which Hontheim took part as an assistant to the electoral ambassador.
Upon Hontheim as auxiliary bishop and vicar-general fell the whole spiritual administration of the diocese; this work, in addition to that of pro-chancellor of the university, he carried on single-handed until 1778, when Jean-Marie Cuchot d'Herbain was appointed his coadjutor.
During the period of his activity as an official at Coblenz he found time to collect a vast mass of printed and manuscript material, which he afterwards embodied in three works on the history of Trier.
These books, the result of an enormous labor in collation and selection in very unfavorable circumstances, entitle Hontheim to the fame of a pioneer in modern historical methods.
The removal of the censure followed (1781) when Hontheim published in Frankfurt what purported to be proof that his submission had been made of his own free will (Justini Febronii acti commentarius in suam retractationem, etc.).