Then will the LORD be jealous for his land, and pity his people.
Then - when God sees His people penitent
Will the Lord be jealous for his land - as a husband jealous of any dishonour done to the wife whom he loves, as if done to himself. The Hebrew х qaanaa' (H7065)] comes from an Arabic root, to be flushed in face through indignation.
Will ... be jealous. The Hebrew is rightly rendered future, as the time that precedes is future-literally, the Lord is jealous; to imply the present instantaneousness with which God will answer His people's penitent prayer (Isaiah 65:24, "Before they call, I will answer; and while they are yet speaking, I will hear").