“but your name will be Abraham.” God changed Abram (“Exalted father”) to Abraham (“Father of a multitude). It was a fairly common custom in the Bible that when a powerful person gained control or fealty from a less powerful person, he changed the name of the less powerful person (Gen. 17:5, 15; 41:45; 2 Sam. 12:25; 2 Kings 23:34; Dan. 1:7). In the biblical culture, the name by which a person was known often said something about the person. It may be about the person’s character, or past, or destiny, but it often (but not always) revealed something about the person. Thus Jacob was “heel snatcher.” Esau was “hairy” and Edom was “red.” Elijah was “My God is Yahweh.” Jesus was “savior.” Abraham was “father of a multitude,” and so forth. Not every name had significance, but most did.
“I have made you.” God speaks of Abraham becoming a crowd of nations as if it had already happened. This is a Hebrew idiom we call the prophetic perfect. By speaking of a future event as if it was past, God promises it will come to pass.
[For more on the prophetic perfect, see commentary on Ephesians 2:6.]