If Christ son of Mary was God or son of God then why?

First of all, the Bible was translated and the exact translation was similar to both teach and lord. second of all, they didn't totally realize that Jesus is God until after His resurrection. you will see that in different occasions in the new test where it's mentioned that they didn't understand what He said until later. But Peter did say to Jesus " You are Son of God" and Jesus said to him you are blessed because no human declared that to you. They were gradually learning and understanding the nature of Jesus.