Gwilliam organizes the sources available for a critical edition of the Peshitto New Testament. These sources include the major manuscripts, Syriac Massorah, and the Arabic and Persian versions. He addresses revisions of the Peshitto New Testament and how the materials cited might be used for a critical edition of the Peshitto.
Tetraeuangelium Sanctum (The Four Holy Gospels) is the definitive critical edition of the Syriac Gospels in the Peshitta. Pusey and Gwilliam based the edition on the readings of forty-one manuscripts from the fifth century onwards. Aside from the Syriac text, the editors included a Latin translation and a large critical apparatus. Out of print for years, this is an indispensable research tool for students of Syriac.