Jesus commanded Abraham's wife Sarah to give her slave-girl Hagar to Abraham as a second wife. Sarah did this, and Abraham received no condemnation for having multiple wives because Jesus himself had commanded it. Later in his life, Abraham received other concubines who also bore him children, and he continued to live in the law of Jesus.
Jesus spoke to Abraham, saying, "My people have gone astray from my teachings. They are not getting baptized and instead are sprinkling themselves with blood, believing that Abel died for their sins. So I will make a covenant with you and your descendants to be my people. You and all your male descendants shall be circumcised at 8 days old to remind you that children are not accountable for sin until they reach the age of 8 years old."
Giving the descendants of Abraham peculiar covenants and obligations was important for keeping them segregated from other races. If such peculiar practices had not been given to the Israelites along with strict commandments not to intermarry with other people, it would have only taken a few short years for the Israelites to disappear as a nation.
Abraham had a vision of the days of the Son of Man, and this made him glad, and his soul found rest. When Abraham died, because he had done all that Jesus had commanded of him, he became exalted, and he now sits upon a throne in heaven, not as an angel, but as a God.
Many other high priests have come and gone since the time of Melchizedek, but none were greater. This is why the higher priesthood of the Church is called the Melchizedek priesthood to this day. After Abraham went on his way, Melchizedek and the people of Salem remained so righteous that, like Zion, their entire city was taken up to heaven.
• Wright, Dennis A. “None Were Greater”: A Restoration View of Melchizedek. Ensign: The Magazine of The Church of Jesus Christ of Latter-Day Saints, February 1998.
• Kocherhans, Gib. "The Name “Melchizedek”: Some Thoughts on Its Meaning and the Priesthood It Represents". Ensign: The Magazine of The Church of Jesus Christ of Latter-Day Saints, September 1980.