If register is an essential part of language description, as has been stressed in past decades, it cannot be left out of consideration in the historic stages of a language, even though we have no direct grasp of its situational-functional contexts there. Although time-consuming and methodologically challenging, this is a necessary undertaking. The paper takes up the challenge, using the examples of Old Lithuanian and Old Latvian, where the beginning of written transmission roughly coincides with the spread of the Reformation and its counter- movements. Our text material is taken from the seven earliest Baltic postils: the Old Lithuanian Wolfenbüttel Postil, Bretkūnas’s, Daukša’s, and Morkūnas’s postils, the first part of Sirvydas’s Punktai Sakymų, and the third part of Knyga Nobažnystės (the Summa), and Mancelius’s Old Latvian postil. Postils, which nowadays are virtually unknown, were a very early, widespread and long-lived book type, so they were perfectly placed within written transmission to influence both the development and change of register. Combining two text types, differing in communication intent and addressees, namely gospel readings and the homilies that explain them, they can be shown to offer an excellent starting point for investigating historical registers, and for possible methods of exploring them.