According to early Christian heresiologists, the Antitheses articulated Marcion’s fundamental convictions based on the separation of the God of the Hebrew Bible from the God of the Christ. While scholars have long stressed the importance of this separation for Marcion’s theology, the genre of the Antitheses, the primary vehicle for this belief, remains problematic. This article proposes that Marcion’s Antitheses ought to be categorized as isagogic, a genre utilized primarily for preliminary instruction. A survey of the salient characteristics of isagogic texts reveals that in form, content, audience, and function, the Antitheses (as described by Marcion’s opponents) align nicely with this genre. Identifying the Antitheses as an isagogic text offers insight into Marcion’s composition and deployment of this work for his evangelic campaign; by leading the reader into a Marcionite interpretation of the scriptures, the Antitheses fostered the catechetical indoctrination and continual growth in the rudiments of his faith.