By RCC do you mean the Catholic Church?
If so why do you misname it?
Jesus is God.
Mary is the mother of Jesus.
Therefore Mary is the mother of God.
Mary is not the mother of God. Mary is the mother of Jesus in in regards to His humanity. How can Mary be the mother of God when the Lord existed in heaven before He came in the flesh?
This does not mean Mary is older than God, or that she is the source of her Son’s divinity. She is the Mother of God in that she carried in her womb, and gave birth to, a divine person – Jesus Christ, God “in the flesh”
Again, how can Mary be the source of her Son's divinity when He existed as God with the Father from eternity? God used Mary as the vessel to bring Christ into the world as a human being. Regarding this, consider what Jesus said to the people:
Then Jesus declared, “How can it be said that the Christ is the Son of David? For David himself says in the book of Psalms:
‘The Lord said to my Lord,
“Sit at My right hand
until I make Your enemies
a footstool for Your feet.”’
Thus David calls Him ‘Lord.’ So how can He be David’s son?”
In other words, how can the Lord Jesus be David's Son, when king David calls Him Lord? The answer is because He existed as God in eternity prior to appearing in the flesh.
It was necessary for the Lord to become a human being so that He could represent us, i.e. be kinned to us, so that through faith in Him who lived a righteous life in the flesh could represent all those would believe in Him and by doing so be credited with His righteousness.
In the same way that through Adam all sinned which leads to separation from God and eternal damnation, likewise through faith in Christ we are credited with His righteousness and reconciled to God which leads to eternal life. This is so, because through our faith God sees us with the righteousness of Christ, even though we still have the sinful nature. When we sin and we confess our sins, God is looking at the righteousness of His Son which is always imputed to us. And Jesus is always speaking on our behalf, making intercession for those who believe in Him.