Exodus 20:5-6, of the Decalogue says, "You shall not bow down to them or worship them; for I, the LORD your God, am a jealous God, punishing the children for the sin of the parents to the third and fourth generation of those who hate me, but showing love to a thousand generations of those who love me and keep my commandments" (NIV). J. I. Packer sees God's jealousy as "zeal to protect a love relationship or to avenge it when broken," thus making it "an aspect of his covenant love for his own people."
–Wikipedia