Exodus 34:14 (also Ex 20:5 Dt 4:24)
Do not worship any other god, for the Lord , whose name is Jealous, is a jealous God.
We usually think of jealousy as a negative thing, such as a person being envious of someone else's belongings or accomplishments, or a husband or wife being unduly suspicious of their spouse and making unreasonable accusations or demands. However, the definition of jealousy gives multiple meanings, including the idea of vigilantly guarding something. I would say that definition is closer to God's jealousy: he is intolerant of our being unfaithful to him (in the sense of our worshiping something or someone besides him), and he is vigilant in guarding our virtue, for it ultimately harms us to worship false gods and not have the benefits of God's love and blessings (see the article on worship).
One may even say that God has a "righteous jealousy," just as he has a righteous anger towards evil. A husband or wife has a right to feel jealous if their spouse cheats on them with someone else; parents who have raised their children well have a right to feel jealous if their children thank a stranger for their upbringing. Similarly, God can have a righteous jealousy when people give to other people or gods the reverence and worship that should be given to him, or offer gratitude to false gods or other people or things for what God has given them.
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