“… the Son of God, the Only-Begotten; begotten of the Father before all ages.”
The difficulty in understanding this phrase is the word “begotten”, a verb no longer used in English. To see what it means, let’s look at an older Bible translation of Matthew 1:2-3:
“Abraham begat Isaac; and Isaac begat Jacob; and Jacob begat Judas and his brethren; And Judas begat Phares and Zara of Thamar; and Phares begat Esrom; and Esrom begat Aram;….”
Now let’s look at these lines in a more modern translation:
“ Abraham was the father of Isaac, and Isaac the father of Jacob, and Jacob the father of Judah and his brothers, and Judah the father of Perez and Zerah by Tamar, and Perez the father of Hezron, and Hezron the father of Ram…”
So to beget means to become the father of. It is somewhat unfortunate that the word is no longer used because it makes clear that Jesus Christ is born of the Father and not made by him. In other words, we can say that like gives birth to like. An animal begets an animal and a human being begets a human being, and God begets a divine Son. The point is, Jesus Christ was born of God (begotten of God) and not made by Him. This crucial phrase is in the Creed, of course, because in the early 4th century AD the priest Arius was teaching that Jesus Christ was made by God, not born of him. In other words, Jesus Christ was a creature made by God. Arius said that Jesus was the greatest creature made by God, but still a creature. In other words, Jesus Christ was a human being and nothing more. So this phrase is present to exclude the false teaching of Arius.
This phrase also defends against adoptionism. This is the idea that Jesus Christ was simply a human being adopted by God at some point, perhaps at his baptism.
Again, this would place Jesus Christ on the same level as the rest of Christians because we become adopted sons and daughters of God at our baptism. In other words, Jesus is always, eternally the Son of God. God begot Jesus before all time in eternity:
“In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God;…” (John 1:1-2)
This means that God lives eternally. Even before the creation of angels, the cosmos and human beings, God the Father had an object of his love in Jesus Christ (and the Holy Spirit). If God is simply an isolated being, what or whom did he live before creation? Believing that Jesus is begotten of the Father eternally, helps us to understand how God is love in his very being from all eternity.