The gospel included in Marcion’s scriptural canon offers the earliest datable extensive witness to the text of Luke, and therefore provides evidence crucial to probing the compositional history of the Synoptic Gospels. This paper surveys that evidence with regard to its implications for the viability of the Two Source Hypothesis. One challenge to the hypothesis is the presence of the so-called Minor Agreements, textual commonalities Luke shares with Matthew but not Mark, and which are not thought to derive from their other supposed common source, the hypothetical Q. The absence from Marcion’s Evangelion of a significant number of the Minor Agreements reveals them to be later harmonizations of the text of Luke to Matthew. Are the remaining Minor Agreements found in the Evangelion to be explained in the same way, as an earlier phase of textual harmonization? Previous scholarship has also identified a significant set of textual agreements between Luke and John in the Passion narrative, likewise lacking in Marcion’s gospel text. Should we regard such harmonizations as a product of copyists or redactors, or can we sustain the distinction, particularly at very early stages of textual transmission? The evidence of the Evangelion, therefore, in some ways addresses challenges to the Two Source Hypothesis, while in other ways adding new complications. Can the clean simplicity of the hypothesis survive what Marcion’s gospel has to tell us?