“And they forsook Yahweh, the God of their fathers, who brought them out of the land of Egypt. And they went after other gods, after the gods of the peoples which surrounded them and they bowed to them. And Yahweh was provoked to anger”
Do you know what it means to forsake something or someone? It means to make a decision to abandon something, to leave it behind intentionally and to renounce your relationship to or connection with that which you have forsaken. The bottom line is that when you forsake someone, they become as if they are dead to you with no grave-marker for remembrance. When you think this through, it ought to give you chills when you think of the people of God doing just that with the God of the heavens who has revealed his power through many works.
May I suggest, though, that anytime God’s people choose to compromise the Word of God, then they are guilty of the same thing. God has revealed himself in his word that we might know him, but oh how quickly entire churches back away from what God has plainly revealed about himself in favor of pleasing the culture and the “itching ears” of those who are listening. When churches reframe God in their own image, then they are just as guilty of forsaking God as these Israelites were. And in turn, their modern apostasy angers God no less than the apostasy of the Jews.
Recognize too, that this is not just a matter of our own personal devotion. What we believe about God is reflected in our witness about God to a fallen world. They are intertwined and inseparable. The Israelites understood this to the degree that they believed that their central creed (Deuteronomy 6:4) was their essential witness to the world. Yet, when we bear false witness against God, we essentially take God’s name in vain (notice that a traditional rabbinic practice is to divide the 10 Commandments in half — five and five — and show parallels between one and six, two and seven, three and eight, etc…). One might be angered if someone committed false witness against them; God pours out his wrath justly for doing so. The Heidelberg Catechism says of the Third Commandment that it is this sin with which God is most angry (Question 100).