The church's earliest theologians unloaded their theological and rhetorical guns on Marcion. The significance of Marcion's position in the history of heresy cannot be overstated. The battle against Marcion was waged to protect not only the Christian canon but also the identity of the God whom Christians worship.
Brevard Childs frames the matter well when he identifies the early church's struggle to conceptualize the Triune God as a struggle for the Old Testament, not against it. Marcion's fundamental misstep, however, wasn't his rejection of the Old Testament, though that was his most notorious act. No, his rejecting the Old Testament was a by-product of a greater problem: his desire to theorize God and his unwillingness to allow the entire canon—Old and New Testaments—to shape his concept of God and thus challenge the deity of his own making.
I bait my students with a provocative turn of phrase from Hans von Campenhausen, a phrase with which I was challenged during my doctoral studies: "The problem in the apostolic era and early church was not, 'What do we do with the Hebrew Scriptures now that Jesus is here?' The problem was actually the reverse: How do we properly understand who Jesus is in light of the assumed authority of the Hebrew Scriptures?"
The force of von Campenhausen's formulation is located at the critical crossroad Christians throughout history have faced when wrestling with the Old Testament canon. How does the Old Testament shape our understanding of God?
There was never a time in the church's history when it operated without a governing canon, despite recent attempts to attenuate this claim by appealing to the uncertain status of the New Testament canon until the fourth century. The Old Testament rendered its Christian subject matter right off the surface of its complex, befuddling, yet Spirit-guided witness to God's Triune identity.
The Triune Solution
The one God who is Father, Son, and Holy Spirit reveals himself to us in the Old Testament as both severe and merciful. The Bible leans against our tendency to construct a god after our own image. We cannot approach the delicatessen of God's person like we approach a buffet—taking a heaping of this and a dollop of that, while passing over what we deem unpalatable. Neither God's severity—"Who can stand before his indignation? Who can endure the heat of his anger?" (Nah. 1:6, ESV)—nor his mercy—"Who is a God like you, pardoning iniquity and passing over transgression?" (Mic. 7:18)—can be diminished or pitted against each other in our reading of the Old Testament. As Habakkuk pleads, "[I]n wrath remember mercy" (Hab. 3:2). C. S. Lewis described Aslan as good but not safe. The same is true of God. Just ask Uzzah (2 Sam. 6:7).
If Marcion's lingering ghost is to be exorcised from the church, it's not enough to simply retain the Old Testament in the Christian canon. Rather, we exorcise Marcion's ghost by identifying the severe yet merciful God of the Old Testament as Father, Son, and Holy Spirit. And it is in the person and work of Jesus Christ in triune fellowship with the Father by the Spirit where the mercy and severity of God meet and are on full display.
I make sense of the whole Bible in this Trinitarian framework. And this sense-making is more than locating Jesus in every nook and cranny—an aggregate attempt to prove his identity as the Messiah and Son of God. Rather, by recognizing God as severe yet merciful, we come to understand more fully who Jesus is. His identity is one and the same with the God of Israel. Reading the Bible in its totality, as God's triune self-disclosure, is the amulet against Marcion's pestering ghost.