I. They congratulate and thank Leo for the Tome.
Having perused your Excellency's letter, which you composed for instruction in the Faith, and sent to the bishop of Constantinople, we thought it our duty, being enriched with so great a wealth of doctrine, to pay our debt of thanks by at least inditing you a letter. For we appreciate your fatherly solicitude on our behalf, and confess that we are the more indebted to your preventing care because we now have the benefit of the remedy before experiencing the evils. For knowing that those remedies are nearly too late which are applied after the infliction of the wounds, you admonish us with the voice of loving forethought to arm ourselves with those Apostolic means of defence.
We acknowledge frankly, most blessed pope, with what singular loving-kindness you have imparted to us the innermost thoughts of your breast, by the efficacy of which you secure the safety of others: and while you extract the old Serpent's infused poison from the hearts of others, standing as it were on the watchtower of Love, with Apostolic care and watchfulness you cry aloud, lest the enemy come on us unawares and off our guard, lest careless security expose us to attack, O holy Lord, most blessed father and pope, most worthy of the Apostolic See.
Moreover we, who specially belong to you, are filled with a great and unspeakable delight, because this special statement of your teaching is so highly regarded wherever the Churches meet together, that the unanimous opinion is expressed that the primacy of the Apostolic See is rightfully there assigned, from whence the oracles of the Apostolic Spirit still receive their interpretations.
Source: Letters (New Advent)