What do we know about the person who finally got this gig?
Not much more than you. I do know that he is the only candidate put forward by Obama to be accepted by the Vatican. According to Christian Telegraph:
Perhaps offering a clue as to what he plans to do as an ambassador, Diaz cited his academic and personal "commitment to public action and service," which he said has inspired him to play "a leading role in promoting cultural, racial, and religious diversity on American college campuses.""At the College of Saint Benedict/St. John’s University, I have worked with religious leaders to [engage] local communities in exploring the role of religion in uniting people. If confirmed, my service to the United States as ambassador would be a natural extension of this work."
The nominee named his experience at "creating dialogues related to cultural diversity, immigration, poverty, and the role of religion in society" as areas of intersection for his possible ambassadorship.
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The pope will find a 'common ground' on which to work and I hope Dr. Diaz is a good listener because Benedict will be giving him an earful on life issues in the US.