I. He complains of the conduct of Dioscorus at the Council of Ephesus
To the most glorious and serene Emperor Theodosius, Leo the bishop.
Already and from the beginning, in the synods which have been held, we have received such freedom of speech from the most holy Peter, chief of the Apostles, as to have the power both to maintain the Truth in the cause of peace, and to allow no one to disturb it in its firm position, but at once to repel the mischief. Since then the council of bishops which you ordered to be held in the city of Ephesus on account of Flavian, does mischief to the Faith itself and inflicts wounds on all the churches— —; and this has been brought to our knowledge not by some untrustworthy messenger, but by the most reverend bishops themselves who were sent by us and by the most trusty Hilarus our deacon, who have narrated to us what took place. And the occurrences are to be put down to the fault of those who met, not having, as is customary, with a pure conscience and right judgment made a definite statement about the faith and those who erred therefrom. For we have learned that all did not come together in the conference who ought, some being ejected and others received: who were ensnared into an ungodly act of subscription by the designs of the aforesaid priest. For the declaration effected by him is of such a nature as to injure all the churches. For when those who were sent by us saw how exceedingly impious and hostile to the Faith it was, they notified it to us.
Source: Letters (New Advent)