There are quite a number of serious sins exposed in this particular issue - perhaps the most serious being their lack of faith in God's promise. There they were, eating "angels' food," as Paul calls it - the very best nutrition they could possibly receive - and it was not good enough! They had to have something extra, just as all the people in the world do, and they were willing to go back into captivity to get it. Is that not compromising? Is that not a lack of self-discipline? Is that not wanting to be like everybody else around them? Is that not seeking after a variety of experiences?
God is interested in unity, in oneness. There is one way, one God, one religion, one set of commandments, statutes, judgments, and so on. But Israel wanted to be like everybody else. They had the best laws, the best country, the best God, the best Husband, but it is not enough. Whenever self-denial becomes an issue, she did not deny herself to serve and submit to her Husband, God. Is that not serious?
God promises to supply our every need, but in Israel's fearful and fickle discontentment, they did not seek Him to understand what He was doing, but instead, they sought something different from what He was providing them within their relationship with Him. We really need to be aware of this, because this drive for fulfillment in variety is still within the Israelitish people, and therefore in us. God shows us in numerous ways that His desire is for His children to be unified in one system, and that is why He told Israel: one God, one place of worship, one system.
They were permitted only one place to keep the Feast of Tabernacles. In my experience in the Worldwide Church of God, people in almost every congregation had to go to a different festival site every year. When we begin to see that this is immaturity, we realize that not everything that God makes available to us is a right choice for us. God shows this right at the beginning of the Book. In Genesis 3, Eve looked at that forbidden fruit and saw that it was good to eat. She could not deny herself the satisfaction of tasting that luscious looking fruit, even though God said not to.
We (especially those of us who live in America, which is far wealthier than any nation has ever been in the history of mankind) have so many things to choose that it is incredible. But what does God say is our responsibility? He says, "Choose life," and there is more to that word "life" than merely being the opposite of death. By "life," He means choose the things that will be good for eternal life.