There is only one Lawgiver and Judge, the one who is able to save and destroy. But you—who are you to judge your neighbor? (James 4:12)
There are a few verses in the Bible that speak of there being "only one" God. Some of them, I believe, were making a statement about monotheism verses polytheism. There is only one God.
But this verse is a little different. There is only one God, he says... and it's not you.
How often do we try to be God? (perhaps much more than we realize.) In this case it seems rather clear--do not judge, for it is only God's place to judge, and you are not He. But what else does God do? He creates, He sustains, he gives life, He grants wisdom, He bestows gifts of the Spirit... have you ever found yourself trying to do any of the above, especially without recognizing that if you have the ability to do any of it, that it is from God?
For example, I try to sustain my life on my own. I have to admit it. I don't always seek God's will for my life but rather I determine what I think it should be and I move on it. (And then I usually ask Him to bless it). But there is only one God of my life, and it is not me. I need to give my life fully to the Lord and ask Him what to do with it.
As do we all. Remember that today. There is only one God, and you are not He. If you think on it long enough, you might just be relieved. :)