In Genesis, we learn that God's word is extraordinarily powerful. Merely through the utterance of a phrase or two -- fiat lux! let there be light! -- things come into being. It is enough for God to say the word, and the earth and the stars and the seas and the forests and countless animals and creatures come to life. Without lifting a finger, merely through the power of a creative verbal act, all the majesty and splendor of this universe takes being thanks to God's word. God is in this way unimaginably powerful. The theological and philosophical term for this attribute of God is omnipotence: the power to do it all, so to speak.
Yet it seems the Bible also recognizes, to some extent at least, a certain weakness of God's word. Whereas the mere command that there be light is enough for light to come into existence, yet in the case of human beings, God's word is not equally powerful. Numerous times throughout the scriptures, starting from Genesis and going all the way through, God speaks to human persons and commands them to do things: not to sin, to love one another, to go here or do that, not to go there nor to do this, etc. But in so many countless cases, the mere fact that God had spoken proved insufficient to bring into being the reality which God had ostensibly sought.
This is true in two ways. In the first place, it is true when God warns people not to commit certain sins or to do evil. At the same time, however, it is true when God speaks in no uncertain terms about the future judgment of evil people. For example, God tells the Ninevites through Jonah that after forty days, they will be destroyed. He doesn't give them the option of repentance, because they admit uncertainty as to whether repentance will do anything to change God's mind (Jonah 3.6-9). Yet God sees that they repent, and so he relents from the destruction he had proclaimed unambiguously. Likewise, in Ezek 33, God tells Ezekiel the following: even if I should speak to a sinner in no uncertain terms that Surely you will die, yet if he repents, I will forget all his sins and he will live.
These are interesting cases because, paradoxically, they seem to be instances in which God's word is powerful enough to undo itself, and thereby to accomplish what God had desired all along. Jonah and God both knew that the Ninevites would repent at hearing the message. I think that is precisely why he tells them in no uncertain terms that they will be destroyed: so that they repent, and God doesn't destroy them. Likewise, God tells the sinner that he will surely die precisely so that the sinner repents, and God no longer punishes him. In such cases, God's word -- mirabile dictu -- accomplishes its purpose precisely by undoing itself and, in a sense, showing itself weak and false.
God shows his power over the natural order through an overpowering word which works infallibly. A rebuke like of a child is enough for Jesus to calm the sea when his disciples feared for their lives. Yet in the case of human beings and their actions, God's word oftentimes seems unable to produce the result it desires. This suggests strongly to me that human beings have a kind of freedom and autonomy relative to God, so that they are not related to God in a purely objective way like the natural order; rather they are irreducible subjects with a life and volition of their own, which God respects. And yet through kenotic self-negating words like in the cases of Jonah and Ezekiel, God's word proves powerful enough to get what it wants precisely while it is undermined by reality.