From Longman Dictionary of Contemporary Englishmissionary zealmissionary zealif you do something with missionary zeal, you do it with great eagerness, because you believe strongly that it is a good thing to do a young English teacher who taught poetry with missionary zeal → missionary
Examples from the Corpus
missionary zeal• Professor Papert deserves high marks for missionary zeal.• The highest court of all is still the Football Association with their brocaded traditions, honest principles and missionary zeal.• Whatever the job at Disney, it was approached with missionary zeal.• A new bureaucracy, the darling of the administration that establishes it, has a missionary zeal about its function.• But it is obvious that one relatively small peninsula can not contain this missionary zeal indefinitely.• The eyes of Mr. Morrissey gleam with a missionary zeal that shames into submission the cringing doubts of those yet unconvinced. 