Langen was born at Cologne, studied at Bonn, and was ordained priest for the Roman Catholic Church in 1859.
His first work was an inquiry into the authorship of the Commentary on St Paul's Epistles and the Treatise on Biblical Questions, ascribed to Saint Ambrose and Augustine of Hippo respectively.
[1] He also published works on the Last Days of the Life of Jesus, on Judaism in the Time of Christ, on John of Damascus (1879) and an Examination of the Vatican Dogma in the Light of Patristic Exegesis of the New Testament.
But he is chiefly famous for his Geschichte der Römischen Kirche (History of the Church of Rome to the Pontificate of Innocent III) (4 vols, 1881–1893), a work of sound scholarship, based directly upon the authorities, the most important sources being woven carefully into the text.
[1] Among other subjects, he wrote on the School of Hierotheus, on Romish falsifications of the Greek Fathers, on Pope Leo XIII, on Liberal Ultramontanism, on the Papal Teaching in regard to Morals, on Vincent of Lerins and he carried on a controversy with Professor Willibald Beyschlag, of the German Evangelical Church, on the respective merits of Protestantism and Old Catholicism regarded as a basis for teaching the Christian faith.