For the Lord to be jealous means that he is zealous for that which belongs to him.1 He will not have his people, who have been created and redeemed by him, serving other gods. Israel belongs to the Lord and God loves his people so much that he wants their undivided love in return. He will not share them with others.2
19 But Joshua said to the people, “You are not able to serve the LORD, for he is a holy God. He is a jealous God; he will not forgive your transgressions or your sins.