Mary of Modena in her court dress in 1680, painted by Sir Godfrey Kneller.
Another voice, the most august of all, was now to break silence. The arguments of Kings, Cardinals, Ambassadors, and of her own family had failed to shake the purpose or convince the mind of the young Princess. Moved by a desire to benefit the Catholics of England, and as much perhaps by the solicitations of the Courts of England and France as by the prompting of his own conviction, Clement X writes a brief to Mary Beatrice, the only instance, we believe, of a Sovereign Pontiff directly addressing a Princess of fifteen years of age. It is, of course, in Latin, but that, as we know, presented no difficulty to her.
James II of England
It is addressed:—
Dilectae in Christo filiae Nobili Puellae Mariae Principesse Estensi. Clemens P.P.X.
“Dear daughter in Christ, noble Damsel, greeting etc. Since the design of the Duke of York to contract alliance with your Nobility reached our ears, We return thanks to the Father of Mercies who, knowing our solicitude for His Glory, is preparing for us, in the Kingdom of England an ample harvest of joy. Considering, in effect, the influence of your virtues, We easily conceived a firm hope that an end might come to the persecution still smoldering in that kingdom and that the orthodox faith, reinstated by you in a place of honor might recover the splendor and security of former days, an effect which no exterior power could accomplish and which might become due to the victory of your piety, the inheritance of your eminently religious family. You can therefore easily understand, dear daughter in Christ, the anxiety which filled Us when We were informed of your repugnance for marriage. For although we understood that it arose from a desire, most laudable in itself, to embrace religious discipline, reflecting that in the present occasion it opposes itself to the progress of religion, we were nevertheless sincerely grieved.
Pope Clement X
We therefore, fulfilling the duties of Our charge, earnestly exhort you by these presents to place before your eyes the great profit which may accrue to the Catholic faith in the above-named kingdom through your marriage, and that inflamed with zeal for the good which may result, you may open to yourself a vaster field of merit than that of the virginal cloister. In order that the special customs and manner of praying, contrary to our ritual, of the populations among whom you are going may be no impediment to your decision, We shall take care that so great an affair be treated in such manner that no obstacle may arise to hinder the exercise of your illustrious piety. Expecting, from the truly filial observance you owe Us, this great consolation, We send your Nobility, from the depth of our heart, our Apostolic Benediction.
“Given at Rome XIX [19th] September MDCLXXIII  the fourth year of Our Pontificate.”
Martin Haile, Queen Mary of Modena, Her Life and Letters (London: J.M. Dent and Co., 1905), pp. 20-21.