Jealousy is one of those words that can be easily misunderstood or used in wrong ways. It may sound strange to our ears to speak of the jealousy of God. What or who makes God jealous? God's jealousy is the jealousy of love for his own people.
Let me tell you a little about the situation God's people were in when he spoke to them the words at the top of this page. God's people, the Israelites, were living among people who worshipped a whole crowd of so-called gods. There were gods of the hills and gods of the valleys, gods of the rivers, gods of thunder, lightning, the sun and the stars. The people who worshipped these false gods made idols representing them and they bowed down to these idols.
God did not want his people to be tempted to join in the worship of these gods - they could not do anything to help them as they were not real, and some of their worship was cruel and horrible. God hated the thought that his people would be caught up in such evil things. He was jealous for them, just as a loving husband would be if he saw his wife going off with another man. God loved them, he cared for them and he didn't want them to be hurt by going the wrong way.
He was also jealous for his own reputation — he is holy and he hated the thought of his name being used in a disrespectful way. God has a holy jealousy for the people he loves and for his name — he and he alone is the living and true God.