False Prophecies of Mormonism Men are Adopted of God

Mormon Doctrine pg 65; “Because of the atonement and by obedience to gospel law men have power to become the sons of God in that they are spiritually begotten of God and adopted as members of his family. They become the sons of God and joint-heirs with Christ of the fulness of the Father’s kingdom. (D. & C. 39:1-6; 76:54-60; Rom. 8:14-17; Gal. 3:1-7; 1 John 3:1-4; Rev. 21:7.)”

Galatians 3:7-8; “Know ye therefore that they which are of faith, the same are the children of Abraham. 8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.”

Why didn’t Mr. McConkie include verse 8 in his example of Galatians chapter 3? Is it because it contradicts Mormon theology? 

How can you be adopted of God and also be literally begotten in the pre-existence?