Idolatry is the most serious evil, b/c it makes 'god' what is not God.
The confidence and faith of the heart can only go to one God alone
Not both God and an idol

Recall the First commandment. In Exodus 20, Moses spends 6 verses on this First commandment:

2 I am the Lord your God, who brought you out of the land of Egypt, out of the house of slavery;

3 Do not have any other gods before me.

4 You shall not make for yourself an idol, whether in the form of anything that is in heaven above, or that is on the earth beneath, or that is in the water under the earth.

5 You shall not bow down to them or worship them; for I the Lord your God am a jealous God, punishing children for the iniquity of parents, to the third and the fourth generation of those who reject me,

6 but showing steadfast love to the thousandth generation of those who love me and keep my commandments.

There is a reason why he spends so much time on this one and it is the same reason why it is the first commandment, it is the most important....

When Jesus was asked which is the greatest commandment in Matt.22:

"You shall love the Lord your God with all your heart, and with all your soul, and with all your mind." This is the great general commandment that Jesus took from the book of Deuteronomy 6:5. And a second is like unto it: "You shall love your neighbor as yourself." All the Law and the Prophets hang on these two commandments.

Israel struggled with idolatry throughout the Old Testament history! And God continually punished them more for this sin than any other sin!