 On 19 February 1645 he became doctor of theology and started to write his first work (1649), against Socinianism.In 1651 he was appointed as minister in Utrecht and on 8 February 1653 he accepted appointment as professor of theology, with an oration "De tractatione verbi Divini".In the academic year 1673-4 Essenius was "rector magnificus" of the University of Utrecht.