I would briefly reconstruct from Genesis, the circumstances leading to this change in names.
In the very beginning of His conversation with Abram, God says to Abram:
Gen: 12:2 (NET)
Then I will make you into a great nation, and I will bless you, and I will make your name great, so that you will exemplify divine blessing.
At this time he was 75 years old and God tells him that his descendent would be numerous (Gen.13:16). But because of his advancing age, Abram asks God that how this can be, when he continues to be childless and whether He means that Eliezer of Damascus would the heir for this to come true. (Gen.15:2-3). In verse Gen. 15:4 God tells Abram that a son born from his own body would be the heir.
But then possibly due to lack of faith in God’s promise, Sarai did not have patience to remain trusted in God and gave their salve girl Hagar to Abram(Gen. 16:2). We can figure out that Ishmael was born because Abraham and Sarah did not trust God to fulfil his promise that their descendants would be from both of them.
Abram was 86 years of old when Hagar gave birth to Ishmael. (Gen.16.16). Thereafter there is no record in the Bible concerning the 13 years of Abram’s life following the birth of Ishmael. After 13 years, when Abraham reaches 99 years of age God instructs Abraham to walk blameless before Him:
Gen. 17:1-2 (NET)
When Abram was 99 years old, the Lord appeared to him and said, “I am the sovereign God. Walk before me and be blameless.Then I will confirm my covenant between me and you, and I will give you a multitude of descendants.”
Here God’s rebuke appears to indicate that during these 13 years, Abram did not walk before God, and as a result, his life was full of blame. Also notice that God re-introduces himself as “God almighty.” God puts a condition to Abram, saying walk before me blameless and only then He will “confirm” His covenant between Him and Abram. Here we see that earlier covenant was not valid for those 13 years and God wants to reconfirm the same again. So God again renews His covenant with Abram:
Gen. 17:4-6 (NET)
“As for me, this is my covenant with you: You will be the father of a multitude of nations. No longer will your name be Abram. Instead, your name will be Abraham because I will make you the father of a multitude of nations. I will make you extremely fruitful. I will make nations of you, and kings will descend from you.
Note here that in Gen. 12:2 it is only one great nation, whereas in Gen 17.4-5 God talks about multitude of nations. Here, God changes the name of Abram to Abraham and Sarai to Sarah. So Abram (Noble Father) becomes Abraham (Father of many) and Sarai, (Princess) becomes Sarah(Mother of Nations).
What is the significance of changing their names? What does it symbolize? Is there any relation of this change to the future coming of messiah and consequently to Multitude of nations (Christianity)?