He published the texts of the two familiar Epistles of Clement in 1875, overlooking the Didache, which he found when he returned to the manuscript.
The words attributed to Jesus on the bread and wine during the Last Supper (found in Mark, Paul, the Didache and arguably in John) and on divorce (found in Mark and Paul) are examples of sayings that are multiply attested.
These include the Epistle of Barnabas, the Shepherd of Hermas and the Epistles of Clement, as well as the Didache.
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.
Furthermore, some theories try to explain the relation of the synoptic gospels to John; to non-canonical gospels such as Thomas, Peter, and Egerton; to the Didache; and to lost documents such as the Hebrew logia mentioned by Papias, the Jewish-Christian gospels, and the Gospel of Marcion.