I. They congratulate Leo on taking the foremost part in maintaining the Faith.
“Our mouth was filled with joy and our tongue with exultation.” This prophecy grace has fitly appropriated to us for whom the security of religion is ensured. For what is a greater incentive to cheerfulness than the Faith? What better inducement to exultation than the Divine knowledge which the Saviour Himself gave us from above for salvation, saying, “go and make disciples of all the nations, baptizing them into the name of the Father, and of the Son, and of the Holy Ghost, teaching them to observe all things that I have enjoined you.”
And this golden chain leading down from the Author of the command to us, you yourself have steadfastly preserved, being set as the mouthpiece unto all of the blessed Peter, and imparting the blessedness of his Faith unto all. Whence we too, wisely taking you as our guide in all that is good, have shown to the sons of the Church their inheritance of Truth, not giving our instruction each singly and in secret, but making known our confession of the Faith in conceit, with one consent and agreement.
And we were all delighted, revelling, as at an imperial banquet, in the spiritual food, which Christ supplied to us through your letter: and we seemed to see the Heavenly Bridegroom actually present with us. For if “where two or three are gathered together in His name,” He has said that “there He is in the midst of them,” must He not have been much more particularly present with 520 priests, who preferred the spread of knowledge concerning Him to their country and their ease? Of whom you were chief, as the head to the members, showing your goodwill in the person of those who represented you; while our religious Emperors presided to the furtherance of due order, inviting us to restore the doctrinal fabric of the Church, even as Zerubbabel invited Joshua to rebuild Jerusalem.
Source: Letters (New Advent)