Why Begotten and Not Made

I think that perhaps C.S. Lewis described it best in Mere Christianity:

We don’t use the words begetting or begotten much in modern English, but everyone still knows what they mean. To beget is to become the father of: to create is to make. And the difference is this. When you beget, you beget something of the same kind as yourself. A man begets human babies, a beaver begets little beavers, and a bird begets eggs which turn into little birds. But when you make, you make something of a different kind from yourself. A bird makes a nest, a beaver builds a dam, a man makes a wireless set—or he may make something more like himself than a wireless set: say, a statue. If he is a clever enough carver he may make a statue which is very like a man indeed. But, of course, it is not a real man; it only looks like one. It cannot breathe or think. It is not alive.

Now that is the first thing to get clear. What God begets is God; just as what man begets is man. What God creates is not God, just as what man creates is not man.

By saying that Jesus is begotten from the Father, we are saying that Jesus is fully God and not a creation of God (Arianism), nor is the Son of God simply a mode or action of God (Sabellianism).