Ah yes — Marcion, the founder of subsequent replacement theologies — erroneously believing that the New Testament Gospel replaced the Old Testament, rather than as Jesus said, fulfilling it.
well, it’s not so hard to see why he went for this — firstly, the canon had not been put together, and just a cursory read of the OT and NT seems to suggest two vastly different gods — an OT God of fire and brimstone who punishes and calls for extinction of peoples and a NT God of love. If we read scripture in bits and pieces and out of context, we can easily fall for the same story that Marcion did.
Just as the Church was the fulfillment of Israel.