If God is almighty and omnipotent, can He commit a sin?
To answer that question, let us divide it into three parts, and define "can", "God", and "sin".
When one asks, "Can ____?" the "can" may be interpreted in at least three ways. For example, "Can we eat mealworms?" The three ways which we understand "can" are: physical freedom, psychological freedom, and moral freedom.
A bird is physically free to do anything it likes. But is it psychologically free, or morally free? Can a bird think about what to eat for breakfast and what for supper? No, because the bird does not possess something called intellect. Intellect is what separates us from the brute animals. Is the bird morally free? No, because it does not know what is right or wrong. It does not have a conscience, nor free will, nor knowledge of good and evil. It cannot make a moral choice.
A prisoner is both psychologically and morally free, but he is not physically free.
What about a normal man, then? A man like you and me. Are we truly free? Yes, but to a limited extent. We are physically, psychologically and morally free to do as we wish, but whether we have the right, whether we may do as we wish, is another question. I may want to feed mealworms to my friends, but if the person does not want to eat mealworms, may I force him to?
After defining "can", we define "God". By definition, God is Almighty, Supreme, Eternal, Omnipotent and Omniscient. He can do all things. Everything.
So we have defined "can" and defined "God". Can God sin?
What is sin? The theological definition of sin is an absence of good. To use an analogy, darkness is an absence of light. Is darkness something? No, darkness is not some-thing that exists in and of itself. Darkness is an absence of light. Where there is no light, there is darkness.
One may argue then, why can't light be an absence of darkness? That is because darkness cannot overcome light. No matter how weak or dim the light, darkness is dispelled. No amount of darkness can overcome how small a flame, simply because darkness is not something. It is a no-thing.
So also for sin. Where there is a lack of goodness, there is sin. Sin is not a something. It is a no-thing. Just as darkness is absence of light, sin is an absence of goodness.
Can God sin? No, because sin is a no-thing, and while God can do everything, it is against His nature to do no-thing. Sin is an absence, it may be said, of God. How can God be without Himself?
Can God sin? No, because it is against His nature.
A related question that is often asked to trick theists is: if God is almighty, can He create a rock so big He Himself cannot carry?
The answer again is no. God indeed can make anything, but the question is self-defeating, and is totally meaningless, because it asks whether God can do a no-thing. It is like asking whether God can draw a square-circle. A square is a square, and a circle is a circle. It is the nature of a square to have four corners, and a circle to be round with no corners. A square-circle is a contradiction in nature, and does not exist. Therefore, God cannot draw a square-circle, and cannot make a rock so heavy He himself cannot carry, simply because the question contains a contradiction in nature.
But as people who profess a Trinity, we can have a little more fun with the rock question. If someone asks whether God can create a rock so heavy He cannot carry, we may say in answer: God the Father may create a rock so heavy, God the Son in the frailty of His humanity cannot carry.Edited 26 Jan 2006: First posted at The Sleepless Eye. - Norman