The vengeful God of the Old Testament is a completely different character from the loving Jesus of the New Testament.

Why people find it persuasive

Old Testament narratives include war, plague, and judgment in ways that read very differently from the Sermon on the Mount, and popular caricatures (sometimes going back to the ancient heretic Marcion) lean hard on this contrast to argue the two are simply incompatible.

Important distinctions

• • Selective sampling of harsh passages vs. the Old Testament's own extensive material on mercy, covenant loyalty (hesed), and forgiveness.
• • Divine judgment vs. divine cruelty — the moral status of judgment depends on justice and proportion, not mere severity.
• • Progressive covenantal unfolding vs. a change in God's actual character.
• • Jesus' own explicit teaching about the Old Testament vs. modern readers' selective impressions of it.

Christian response

• • Point out this reading requires ignoring large portions of the Old Testament itself: Exodus 34:6 describes God as 'merciful and gracious, slow to anger, abounding in steadfast love and faithfulness,' and this exact formula is repeated and celebrated throughout the Psalms, Prophets, and Wisdom literature, making mercy, not just judgment, a central Old Testament theme.
• • Point out symmetrically that the New Testament, including Jesus himself, contains extensive judgment material — warnings about hell, the cleansing of the Temple, woes pronounced on religious leaders, and the severe imagery of Revelation — so 'wrathful Old Testament vs. gentle New Testament' collapses once both testaments are read in full rather than through selective quotation.
• • Bring in that Jesus explicitly affirms the Old Testament as authoritative Scripture revealing his Father (he quotes it as final authority, calls it unbreakable in John 10:35, and identifies the God he calls Father with the God of Abraham, Isaac, and Jacob), which is very hard to square with the idea that he saw it as depicting a different or inferior deity — a move the early church explicitly and forcefully rejected when Marcion proposed exactly that in the second century.
• • Explain the Christian theological framework of progressive revelation: the same just and merciful God relates differently to humanity across different covenantal stages (the establishment of Israel as a nation among nations facing real historical threats, versus the inauguration of a kingdom that transcends ethnic and national boundaries), without that difference in historical mode implying a difference in the character of God.
• • Address the hardest cases (like the conquest narratives) honestly rather than deflecting: these remain the most morally difficult texts in the Old Testament, and serious Christian scholars disagree on how to read them (as hyperbolic ancient Near Eastern conquest rhetoric, as a unique and non-repeatable episode of covenant judgment, or otherwise), rather than pretending there is a fully comfortable resolution.

Christian counterresponse

Christian theologians respond that this shift is explained by Israel's unique role as a geopolitical nation under direct covenant in a specific historical and redemptive-historical moment, a status the church does not inherit in the same form (the New Testament explicitly reorients holy war language into spiritual rather than military terms), so the absence of a New Testament military analogue is expected by the theology itself, not an unexplained anomaly.