Ezekiel 23:30
I will do these things unto thee, because thou hast gone a whoring after the heathen, and because thou art polluted with their idols.
Cross-reference
Ezekiel 23:7 details how Oholibah defiled herself with Assyrian lovers and idols, specifying the harlotry this verse condemns.
Ezekiel 23:12-21 details the very 'playing the whore after the nations' that verse 30 punishes — direct internal reference.
Ezekiel 23:17 recounts her affair with Babylonians, showing a specific instance of the defilement punished in verse 30.
In Ezekiel 6:9, God mourns Israel's 'whoring heart' after idols — same metaphor of spiritual adultery.
Psalm 106:35-38 describes Israel mixing with nations and serving idols — the exact sin Ezekiel 23:30 condemns.
Jeremiah 2:18-20 rebukes Israel for pursuing Egypt and Assyria, 'playing the whore' — identical condemnation of political idolatry.
Jeremiah 16:11 says fathers forsook God to serve other gods — the same root sin as 'playing the whore after the nations'.
Jeremiah 22:9 gives the same reason for judgment: forsaking the covenant and worshiping other gods, confirming the cause of the harlotry described here.