A major theme throughout the Torah, and specifically in this parshah, is the idea that idolatry is bad. Real bad. Bad enough to receive the death penalty. In a case mentioned here, bad enough where if a whole city (Jewish that is) that falls under the spell of idolatry everyone must be killed and all of the property has to be destroyed. Ignoring the fact that it is nigh impossible to receive the Biblical death penalty because of the high standard of proof, this still seems unfair. Why can’t the Bible just be more tolerant? If G-d is really G-d, why would he get jealous of other gods?
This is the big deal: religion, whatever or however you want to define it, is the operating principals behind your life. It’s the package of ideas that you consider axiomatic. These principles ultimately guide your behavior and your understanding of right and wrong. A wrong understanding of the world can have disastrous ramifications for your entire moral conduct. It’s like this: if your understanding of table manners are messed up, you will eat like a pig; if your entire concept of morality is off, anything is up for grabs.
Of course the more tightly formulated your religious concept is, the tighter this conception of how the world operates is going to be. The Jewish concept is very tight. When the Torah says that G-d is jealous, He isn’t jealous. He’s zealous, exacting (it’s the same word in Hebrew but in English there’s several things קנא can mean). G-d sets a high bar for Jews. We are supposed to be a model of behavior for the world. The easiest way to dump out of that responsibility is to just trade in the G-d of the Jews for some other god and do as you please. That’s why idolatry is considered to be so serious.