Not sure where you derive that from. Wasn't this your post?
That just isn't true--and can be shown as a false claim by reading Genesis 12:
Genesis 12---King James Version
1 Now the Lord had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee:
2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:
3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
4 So Abram departed, as the Lord had spoken unto him; and Lot went with him: and Abram was seventy and five years old when he departed out of Haran.
There is the condition, and God's grace. Abram obeyed. Abraham's obedience is listed as the reason of the promise:
Genesis 26:4-5---King James Version
4 And I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries; and in thy seed shall all the nations of the earth be blessed;
5 Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.
That's what one will find being taught in the LDS church--God's grace for our obedience to Him--just as Genesis shows.
Josheb--your claim -- "The first mention of the covenant with Abraham we have is in Genesis 15"--is a false claim, regardless of what you believe about whether God had already chosen Abram or not.
That occurs in Genesis 12--and is connected to Abram's obedience