unfaithfully: See Ezekiel 6:9; 16:17; Hosea 2. Jerusalem had turned away from the Lord and followed idols instead of serving the true God. This was spiritual unfaithfulness. The prophets often describe this sin as breaking faith in a marriage relationship.
By worshiping idols, the people broke the covenant (the binding promises between God and his people). Because they broke this covenant, judgment would follow.
As a result, the land would become desolate (empty and ruined).