1. The catalyst for Nicea was Arianism which taught that Jesus was not fully divine. This strikes at the heart of the gospel.
2. Jesus must be fully divine (fully God) in order to accomplish the divine work of salvation. He had to be God to save man from the punishment/wrath of God against sin.
3. The key phrase in this debate is "only begotten"
4. Scriptures focus of Christ is redemption - keeping everything in context is key. By keeping the focus on the context of Jesus redemption of all mankind - the church fathers were able to stay true to scripture and not get caught up in a philosophical and semantic rabbit trail. It focused them on the work at hand.
5/6. The serious mistake or weakness in Arian theology is a mistake in categories. Arianism projects human categories onto God. Because God is eternal, the relationship with Jesus is eternal. Jesus is eternally begotten of His Father. Jesus is "of" the Father in a unique way that all other creation is not because Jesus is not created - He IS God, the creator. John 1:1-3 states In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made.
7. The fathers of Nicea "anchor" their teachings about Jesus to the redemptive context of scripture. The Bible speaks of the Son as fully God, and since the Bible speaks of the Father as God, the phrase "only begotten" must be interpreted in this context.
8. The fathers at Nicea used a very specific word to clarify all of this, homoousios which is translated in English "of the same essence". Whatever one understands to be that which constitutes deity - it is the same for the Father and for the Son.
9. God gives Himself, in Christ Jesus, to save sinners. This is the truth and power of the gospel. We are so loved that God gave Jesus for our redemption.
Romans 5:8 But God demonstrates his own love for us in this: While we were still sinners, Christ died for us.John 3:16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
John 14:20 "On that day you will realize that I am in my Father, and you are in me, and I am in you."