The Lord highlights Judah’s unfaithfulness by addressing the nation in the form of a disputation in which he asks a question and follows this with an accusation (see Jeremiah 2:32; Jeremiah 3:1–5; Jeremiah 8:4–5; Jeremiah 18:14–15).1
The two questions he asks about whether an Ethiopian can change the colour of his skin, or a leopard his spots both have the obvious answer of, No.
In the same way Judah is simply unable to do good. The nation is unable to be a faithful covenant partner. Their long history of unfaithfulness and idolatry means that this state of rebellion has become permanent. The human heart is sinful by nature, but sin and rebellion are also nurtured and reinforced by years of repetition.2
23 Can the Ethiopian change his skin or the leopard his spots? Then also you can do good who are accustomed to do evil.