 During this time, Grotius returned to the theological issues which had earlier caused him such grief. The project of Christian unity—harmonizing both the various Protestant factions and the Protestants with the Catholics—became increasingly important to him. Both intellectually and practically, it was a task which suited him and his place in life: intellectually, because Christian unity raised many conceptual puzzles which challenged his mind; practically, because as ambassador for one of the great European powers, he could use his position to press the cause. Of course, the cause was doomed to failure—certainly, it was well beyond Grotius’ abilities. Yet, it did lead him to write some extremely interesting and influential works, many of which were collected in his Opera Omnia Theologica.