9:23-10:3 clearly display just how important it is to acknowledge God’s holiness through obedience. The consequences of obedience in 9:23ff is clear–God appeared to them and was pleased (burned up the sacrifices) and the people were filled with joy. The severe consequences of disobedience 10:1-3 is shocking. Yet this clearly expresses just how holy God is and how very important it is to obey him.
One of the most important overarching principles of how God acts in the Old Testament is tied intimately to the New Testament. God acts in severity in the OT because he is in the process of implementing his plan of salvation for all people (old and new, ancient and modern) that is fully revealed in the NT. This principle is called “the cradle of salvation.” In the OT God is bringing to reality is promise of Genesis 2 that God would provide a way for humankind to be reconciled (“he will crush his head [the Messiah's victory over death], and you will strike his heal [Jesus' suffering the cross]“. The severity of God’s judgment is a result of his protecting his plan of salvation for the whole world! God had promised that the “seed” would come through Abraham–through Israel. So to keep this motley group of people together as a nation long enough to bring about the promised Messiah was essential. If they messed up and split up the promise was at risk. So God gave them laws and governed them severly to make sure that his promise would be realized. In the process, Israel saw and we see that God is not to be taken lightly. There are great blessings waiting for those who hear and obey but also judgment for those who do not.
Over and over the phrase is repeated, “I am the Lord.” This is the reason for all that we should do and the justification for all that God does. He is Lord. There is no other. The simple choice is to believe and follow or not believe and go our own way with staggering consequences.