Chapters 18-20 tell of some of David’s military victories. In chapter 21, we are told of what seems like a strange failure on David’s part. David conducts a census. God sees this as rebellion because David is depending on how many soldiers he has rather than on God. God’s punishment extends to all of Israel and is stopped by David sacrificing offerings on an altar. The location of that altar will eventually become the location of the Temple.
When we think of sin we think of the adultery David committed. We don’t always think of depending on things other than God. For David it was military might. For us it could be our investments, possessions, our nation’s military or the doctors we see. It’s good to make plans to take care of ourselves, but ultimately, our dependence is on God.