The Second Person in God, the Son, became human Himself: was born into the world as an actual man—a real man of a particular height, with hair of a particular colour, speaking a particular language, weighing so many stone. The Eternal Being, who knows everything and who created the whole universe, became not only a man but (before that) a baby, and before that a foetus inside a Woman’s body. If you want to get the hang of it, think how you would like to become a slug or a crab.
From Mere Christianity
Before God fathered Jesus, His only begotten Son, our Savior existed as God’s creative Word. I think Lewis understated his metaphor, though; the picture of a human lowering himself to become a slug is simply not low enough. My mind pictures a human being becoming a bacterium for the sole purpose of eradicating the fatal, Beelzo transgvirus-1, also known as the sin virus.
Lots of folks have trouble with the Biblical statement that Jesus was the only begotten of God, due to their misunderstanding of Trinitarian doctrine. The alleged paradox lies in the idea that Jesus, God’s divine Son, had a beginning, and as God is eternal, that couldn’t be true. As a clever workaround, wordsmiths came up with the phrase, “begotten, not made,” in the Nicene Creed, but they needn’t have gone to all that trouble. The truth is far simpler; Jesus of Nazareth, though He was indeed born in the flesh, was not born of the flesh, because His Father is God Himself. As such, Jesus is the perfect man, the second Adam, who unlike the first Adam, never rebelled against His Father.
In other words, God the eternal Word became a mortal man at Jesus’ conception, enabling Him to be at once, immortal and mortal.
I hope that clears up the “only begotten” aspect of Jesus’ incarnation, so you’ll know the simple answer next time someone asks you the hard question.