Included among the topics on which he wrote are: the Apology of Aristides (1891), the Didache, Philo, the Diatessaron, the Christian Apologists, Acts of Perpetua, The Odes and Psalms of Solomon (1906), the Gospel of Peter, and other Western and Syriac texts, and numerous works on biblical manuscripts.
Other important works are the Evangelienbuch (Gospel harmony) of Otfrid von Weissenburg, the short but splendid Ludwigslied and the 9th century Georgslied.