After devoting more than twenty years to this undertaking, Coustant was able to publish the first volume in 1721. It contains the letters from the year 67 to the year 440, and is entitled "Epistolae Romanorum Pontificum et quae ad eos scriptae sunt a S. Clemente I usque ad Innocentium III, quotquot reperiri potuerunt...""(Paris 1721). In the extensive preface perhaps of 150 pages Coustant explains the origin, meaning and extent of the papal primacy and critically examines the existing collections of canons and papal letters.