Neither shall your name any more be called Abram, but your name shall be Abraham; for a father of many nations have I made you.
Read Chapter 17
Ambrose of Milan
God changes Abraham’s name, adding one letter. Instead of Abram he is called Abraham, that is, instead of useless father—such is the interpretation of the name—he is called sublime father, chosen father; or, alternatively, from being simply father he becomes father of a son. He was useless because he did not know God. He was made the chosen one after he had come to know God. He was father when he had had offspring through the slave girl, but he was not father of a son, because he was not truly his son who had not been born of a legitimate marriage. When Sarah gave birth, he became father of a son. . ...
However, a question arises here which should not be passed over and which may perhaps also, quite independently, be bothering some of you. What does it mean, that when the name of Abraham, this man Jacob’s grandfather, was changed (he was previously called Abram, you see, and God changed his name and said, you shall not be called Abram, but Abraham)? From that time on he was never called Abram. Search in the Scriptures, and you will see that earlier on, before he received another name, he was only called Abram. After he had received the new one, he was only called Abraham. This man Jacob, however, heard the same words when he received another name: You shall not be called Jacob, but you shall be called Israel. Now search the Scriptures, and see how he was always called either name, both Jacob and Israel. When Abram got another name, he was never called anything but Abraham; when Jacob got another name, he was called Jacob and Israel. The name Abraham was to receive its explanation in t...
Abraham. Abram, in the Hebrew, signifies a high father; but Abraham, the father of the multitude: Sarai signifies my Lady, but Sara absolutely Lady. (Challoner)
God thus receives them as it were into his own family. (Calmet)