What does it mean that God is a jealous God?
I know that it can seem confusing that God could be completely loving and jealous at the same time. In the context of Exodus 20:4-6, God is not using jealous as you or I would. We use jealous to describe the feeling we get when someone else has something that we want. This verse is a description of the forth commandment which says, “Thou shalt not make up for yourself an idol in the form of anything in heaven above or on the earth beneath or in the waters below. You shalt not bow down to them or worship them; for I, the Lord your God, am a jealous God...” He is saying that He will not tolerate one of His creation worshipping or bowing down to a God other than Him.
I think the word fits perfectly because if you read on, it is clear that there is no gray with the Lord. “…punishing the children for the sin of the fathers 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.” He will bless those who love Him and punish those who don’t, but He is jealous for, or longs for, the affections of all people. And take notice, He is far more lavish with His rewards than He is longlasting with His punishments. In particular, the references to God as a jealous God draw upon the human emotion and use it as a metaphor to express something about God. To leave it on the literal level assumes the human emotion, which is almost always negative, is a description about how God "really is." However, to take this as an anthropomorphic description about God allows it to move beyond the negative human emotion. The Israelites existed in a world and among people who believed in the worship of many gods.
Their greatest religious threat came from Canaanite fertility religions that personified the forces of nature into deities (see Ba’al Worship in the Old Testament). The Canaanites could add Yahweh, the warrior God of the Israelites, to their worship as another of the nature Gods. But the God of the Israelites was not just another god, He was THE God. The Israelites recognized Him as the only Sovereign Creator God. The metaphor used to make this claim was the metaphor of God as a jealous God. He would not simply be added to the other nature deities, because he had created the aspects of nature that the Canaanites worshipped!
Hosea takes this metaphor and makes it positive, as he compares God’s relationship with Israel in the imagery of a marriage. The basis is love, and "jealousy" is a way to describe the love that a husband has for a wife who is unfaithful and works as a prostitute. This "jealousy" is one that hurts because of the unfaithfulness of the wife (Israel) in a relationship in which she had pledged herself to her husband who had loved and cared for her (God). So when biblical writers apply this term to God, they are affirming that He, as Creator and Deliverer, is the only God worthy of our worship.