There's an interesting scene in King David's life where he comes under tremendous curse from god for taking a census without being commanded to do so. It seems strange to us, a sever overreaction on God's part. After all, there seem to be a lot of valid reasons why a king would take a census of his own people.
Israel, at the time, was fighting a good deal of battles. Perhaps David needed to know how many fighting men he had left. Or how many fighting men he had lost. David was resettling and restructuring the kingdom of Israel, establishing his own palace and a firm capital in Jerusalem. Perhaps he needed to understand the logistics of governing such a vast people from a central location. Maybe David just didn't trust Saul's numbers and wanted to have some of his own. There's much to be known about the people you're governing from simply counting them.
But it wasn't simply a count.
To understand why David's unapproved census was such a big deal, we have to turn all the back to Exodus 30, to the first counting of the Israelites. Check out God's instructions for taking the census:
When you take a census of the Israelites, each person must pay the Lord a ransom for his life when he is counted. ...As each person is counted, he must give one-fifth of an ounce of silver using the standard weight of the holy place. ...This contribution is given to make peace with the Lord and make your lives acceptable to the Lord.
Aha. All of a sudden, David's sin starts to make sense. On many levels.
When David takes an unauthorized census of his people, he is levying a tax on them, basically. He's making them pay money in order to be counted among his kingdom. Because you can be sure that the king is going to do the census the way the census is said to be done - collecting the money - even if it's never called for. He's doing what we all do and combining a little bit of God's idea with a little bit of his own idea and trying to make it a thing. So on one level, he's forcing his people to pay something at a time when they shouldn't have to. This is a sin.
But it's not just a payment they're making. Look again at what it says in Exodus - this payment is a ransom. Since when is it David's job to decide when the people of God must be ransomed to God? Now, not only is David requiring a payment from his people without authorization, but he's requiring a religious act from them without authorization. Slowly, he seems to be stepping into the role of God.
And perhaps even more than that. Because maybe David isn't asking for the ransom money to go to the Lord. Maybe David is putting this ransom count in his own treasury. Maybe he's investing it in his own army. After all, it's his census, not God's. Shouldn't he get the spoils from the people's burden of being counted? If this is the case, David is attempting to ransom his own people somehow, and this certainly puts him in the place of God in this whole dynamic.
Now, God's reaction to David's unauthorized census doesn't seem quite so overzealous. God has every right to be upset with this whole situation - because David is doing something to God's people that God has not approved of (levying a financial burden on them) and because David is subtly slipping into the place of God himself by doing so.
And I think the people probably had a right to be upset, too. Imagine if you were one of the people. You knew the procedures for having a census. You knew generally when and how the censuses came. And out of nowhere, the king calls another census. You have to scrape together one-fifth of an ounce of silver (which Exodus indicates is a bit of a median value, probably - the rich would have had much more, the poor would have struggled to have this much), silver that you may have set aside for something else. And you have to give it to the king. Not the Lord, but the king. Imagine the resentment you might feel at what the king is doing to you.
So here is yet another layer of David's sin - he's creating a resentment in the heart of the people for their king. And their king is the king that God has chosen for them. If the people resent this king, they will soon come to resent this God who elected this king.
This whole scenario shows a couple of important things, but perhaps the most important is this: even the little things we do may not be so little after all. We have to be very conscious of the implications of our actions, and that comes from knowing what those actions mean. It doesn't seem like much of a thing that David wanted to count his people. But when you understand that counting his people meant taxing them, ransoming them, stepping into the role of God, and creating a resentment among them...it's absolutely a grievous sin.
The question then becomes: what little thing are you thinking about? And what if your little thing is actually a big thing?