Question: "Why is God a jealous God?"
Answer: It is important to understand how the word "jealous" is used. Its use in Exodus 20:5 to describe God is different from how it is used to describe the sin of jealousy (Galatians 5:20). When we use the word "jealous," we use it in the sense of being envious of someone who has something we do not have. A person might be jealous or envious of another person because he or she has a nice car or home (possessions). Or a person might be jealous or envious of another person because of some ability or skill that other person has (such as athletic ability). Another example would be that one person might be jealous or envious of another because of his or her beauty.
In Exodus 20:5, it is not that God is jealous or envious because someone has something He wants or needs. Exodus 20:4-5 says, "You shall not make for yourself an idol in the form of anything in heaven above or on the earth beneath or in the waters below. You shall not bow down to them or worship them; for I, the LORD your God, am a jealous God..." Notice that God is jealous when someone gives to another something that rightly belongs to Him.
In these verses, God is speaking of people making idols and bowing down and worshiping those idols instead of giving God the worship that belongs to Him alone. God is possessive of the worship and service that belong to Him. It is a sin (as God points out in this commandment) to worship or serve anything other than God. It is a sin when we desire, or we are envious, or we are jealous of someone because he has something that we do not have. It is a different use of the word "jealous" when God says He is jealous. What He is jealous of belongs to Him; worship and service belong to Him alone, and are to be given to Him alone.
Perhaps a practical example will help us understand the difference. If a husband sees another man flirting with his wife, he is right to be jealous, for only he has the right to flirt with his wife. This type of jealousy is not sinful. Rather, it is entirely appropriate. Being jealous for something that God declares to belong to you is good and appropriate. Jealousy is a sin when it is a desire for something that does not belong to you. Worship, praise, honor, and adoration belong to God alone, for only He is truly worthy of it. Therefore, God is rightly jealous when worship, praise, honor, or adoration is given to idols. This is precisely the jealousy the apostle Paul described in 2 Corinthians 11:2, "I am jealous for you with a godly jealousy..."