 In 1425 he entered the service of the archbishop of Trier and, as his secretary, received income from several benefices. By 1426 he was in Cologne where he may have lectured on canon law, studied philosophy and theology, and began researches into original source material and into the annals of German law. There he was able to prove that the Donation of Constantine was an eighth-century forgery, and he found manuscripts of Pliny’s Natural History and of Plautus’ comedies. In Cologne he met Heimerich of Campo, who introduced him to the writings of Ramon Llull, Albert the Great and his commentary on Dionysius, and Proclus’ commentary on Plato’s Parmenides. In 1428 and 1435 Nicholas was offered positions teaching canon law at the University of Louvain, but he turned them down to remain in church administration.