St. Leo's most famous writing is the Tome of Leo, a must-read for anyone interested in Christology and the Incarnation. You can also read some of his sermons. From Sermon 95:
"Blessed are the peace-makers, for they shall be called the sons of God." This blessedness, beloved, belongs not to any and every kind of agreement and harmony, but to that of which the Apostle speaks: "have peace towards God;" and of which the Prophet David speaks: "Much peace have they that love Thy law, and they have no cause of offences." This peace even the closest ties of friendship and the exactest likeness of mind do not really gain, if they do not agree with God's will. Similarity of bad desires, leagues in crimes, associations of vice, cannot merit this peace. The love of the world does not consort with the love of God, nor doth he enter the alliance of the sons of God who will not separate himself from the children of this generation Whereas they who are in mind always with God, "giving diligence to keep the unity of the Spirit in the bond of peace," never dissent from the eternal law, uttering that prayer of faith, "Thy will be done as in heaven so on earth." These are "the peacemakers," these are thoroughly of one mind, and fully harmonious, and are to be called sons "of God and joint-heirs with Christ," because this shall be the record of the love of God and the love of our neighbour, that we shall suffer no calamities, be in fear of no offence, but all the strife of trial ended, rest in God's most perfect peace, through our Lord, Who, with the Father and the Holy Spirit, liveth and reigneth for ever and ever. Amen.