How could Jerusalem have fallen? The surviving Judeans (aka Israel) had these promises from God, written down in the Law:
Ezekiel 33:24 Son of man, the scattered remnants of Israel living among the ruined cities keep saying, ‘Abraham was only one man, yet he gained possession of the entire land. We are many; surely the land has been given to us as a possession.’
This could have been seen as an arrogant claim to an undeserved inheritance. God’s answer was that Israel needed to look at itself: was there anything that a reasonable person would expect a just God to protect?
Ezekiel 33:25-26 So tell these people, ‘This is what the Sovereign Lord says: You eat meat with blood in it, you worship idols, and you murder the innocent. Do you really think the land should be yours? Murderers! Idolaters! Adulterers! Should the land belong to you?’