The Only Begotten Son Language. The second piece of evidence we must examine is the expression only-begotten. It is the Greek word monogeneis. This is not simply begotten, for that expression can be applied to all believers, those who have been begotten or born again by the Spirit. This is a unique expression for a unique person, the only-begotten Son of God. The expression appears in John 1:14, 4:18, 3:16, and 3:18. It would literally mean the only generated one. This is the key expression for the doctrine of the eternal generation of the Son, meaning, he always was the only begotten Son. The expression does not refer to the birth of Jesus in Bethlehem, because he is the Son from eternity past.
Perhaps the language can be better understood if contrasted with synonyms. Take the verbs make, create, and beget. The verb make is general; one can make dinner, clothes, a house, or any other product. The create can have the same objects, but usually elevates the act to an art: one creates a masterpiece, or a work of art, or a symphony. While these creations bear the imprint of the creator, they do not share his nature. But beget is different. You can only beget a child that has the same nature as you have--a son or a daughter. There is nothing else you can beget (unless you were speaking very figuratively). Your son or your daughter will inherit his or her nature from you--genes, personality--all of it. You can use make or create for producing a child; but when you use beget it only means you produce a child that has your nature.
Now follow this carefully. If Jesus is said to be the begotten Son of God (using the figure from human language to make the point), then Jesus has the same nature as the Father. If Jesus has the same nature as God the Father, then Jesus is divine and eternal as well. If he is eternally God, then there was never a time he was literally begotten--which is why we know the language is figurative to describe his nature, and not his beginning. To call Jesus the only begotten Son means that he is fully divine and eternal. He is God the Son.
This is why the creed says that Jesus was begotten, not made. Why? Because he is of one substance with the Father.
One more point. The word begotten has only (mono-) prefixed to it. There is only one. This means that Jesus has a unique relationship with the Father--they two along with the Holy Spirit make up the Godhead. You and I, if we are believers, have been born into the family of God--we are said to be begotten of God. But we are not only-begotten. That refers to Jesus divine nature. We were adopted by grace and given the divine nature by the Spirit so that we may be called the children of God. But Jesus--he is very God of very God. He is the only-begotten Son of God (that is the part of the creed that reads of very God), which means that he is God (that is the part that reads very God).