Problem with my thesis, what should I do?

I am French, from an affluent family of aristos. From my early age I wanted to be a cleric, and studied in a seminary. Just before I was to graduate, I had to leave it (long story, not relevant), and, as one can't be idle, I joined the army. I was twenty years old then. I have been in the army for some years, but always thought it was only temporarily. So, when I was wounded in my last assignment, I turned back to church and decided to write my thesis and, at last, be ordained. I am now at Crevecoeur, and Monsieur the Principal of Amiens and Monsieur the Curate of Montdidier are helping me to decide on the subject. Monsieur the Principal thinks that my thesis should be dogmatic and didactic, and I would rather it should be ideal. Monsieur the Principal proposed this subject: "Utraque manus in benedicendo clericis necessaria est" (The two hands are indispensable for priests of the inferior orders, when they bestow the benediction). I think it's admirable, yet it requires a profound study of both the Scriptures and the Fathers, which I somewhat neglected in the army. So, I chose the different subject, 'Non inutile est desiderium in oblatione" that is, "A little regret is not unsuitable in an offering to the Lord." But, Monsieur the Principal was really against it, he said it was too close to the heresy of Pelagians, and that, at best, it would have the success of Monsieur Patru, an Academician, this profound genius of French language and elegant speaking style! In short, we can't agree on it, and I am eager to start, what should I do?