This photo explains a little bit more on the Immaculate Conception of Mary.
Remember – everything comes from God, our Creator. To believe Christ is God is to be a Christian. Christ is God. He is not just a prophet. Mary was saved by God (Jesus.) God is outside of time. God chose Mary out of the billions of women throughout time to be His mother.
When the Holy Spirit overshadowed Mary, a virgin, she said yes to His will, and was pregnant with baby Jesus. Joseph was not Jesus' natural father, the Holy Spirit (God) was. Christians believe this, as unbelievable as it seems. The Angel told Mary she was "full" of grace. Think about it. To carry God in your womb, you would have to be immaculate and without the stain of sin. God is pure goodness, pure holiness. He would not have chosen to grow and develop in a womb for 9 months that was not completely pure.
Catholics call Mary the "Ark of the New Covenant". In the Ark of the Old Covenant, God came to his people with a spiritual presence, but in Mary, the Ark of the New Covenant, God comes to dwell with his people not only spiritually but physically, in the womb of a specially prepared Jewish girl. God knew Mary before she was born, and chose her to carry Christ. Behold your Mother. He gave her to us.