15 And God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name.
16 I will bless her, and moreover, I will give you a son by her. I will bless her, and she shall become nations; kings of peoples shall come from her.”
17 Then Abraham fell on his face and laughed and said to himself, “Shall a child be born to a man who is a hundred years old? Shall Sarah, who is ninety years old, bear a child?”
18 And Abraham said to God, “Oh that Ishmael might live before you!”
19 God said, “No, but Sarah your wife shall bear you a son, and you shall call his name Isaac. I will establish my covenant with him as an everlasting covenant for his offspring after him.
20 As for Ishmael, I have heard you; behold, I have blessed him and will make him fruitful and multiply him greatly. He shall father twelve princes, and I will make him into a great nation.
21 But I will establish my covenant with Isaac, whom Sarah shall bear to you at this time next year.”
22 When he had finished talking with him, God went up from Abraham.
23 Then Abraham took Ishmael his son and all those born in his house or bought with his money, every male among the men of Abraham’s house, and he circumcised the flesh of their foreskins that very day, as God had said to him.
24 Abraham was ninety-nine years old when he was circumcised in the flesh of his foreskin.
25 And Ishmael his son was thirteen years old when he was circumcised in the flesh of his foreskin.
26 That very day Abraham and his son Ishmael were circumcised.
27 And all the men of his house, those born in the house and those bought with money from a foreigner, were circumcised with him.
Romans 4 makes it clear that Abraham’s laughter in v. 17 was the laughter of faith and not of unbelief. It was the joyful laughter of a worshiper when Abraham fell upon his face. His words in effect were, “Oh what joy, Sarah and I, though so aged, are to have a child!” The Lord in John 8:56 no doubt pointed to this occasion when He said that Abraham rejoiced to see His day and was glad. The exclamation, “Oh that Ishmael might live before thee” was not the cry of unbelief—as is plain from v.20—nor was it a prayer that Ishmael should be the child of promise, but it was a cry of faith that Ishmael might receive some measure of Divine blessing, though he was to be set aside in favor of the unseen child now promised.— Williams, pages 21-22.
Sarah (v.15) = princess
Abraham was so elated at God’s promise that he laughed with joy and surprise. That it was not a laugh of doubt is evident from the fact that God gave him no rebuke, as He later did Sarah when she laughed (Genesis 18:13). The questions which Abraham asked likewise were not in doubt, but in wonder and happy amazement.
Then he remembered Ishmael, and it seemed as though God’s new promise would cut Ishmael altogether out of His favor. He therefore interceded for Ismael, desiring God to bless him as well.
Yes, God assuredly would bless Ishmael too; but first He emphasized again to Abraham that His covenant was with Isaac alone, and with his seed. In recognition of Abraham’s joy, God told him to name his son Isaac (meaning “laughter”). He also gave him the glad news that Isaac would be born in only one year. …
Even though Ishmael was not to inherit the promises with Isaac, Abraham rightly desired to have him included among those receiving the spiritual blessings that would stem from the fulfillment of those promises. All this was done on the same day God had spoken to him. This required a particular act of faith on Abraham’s part, since it no doubt incapacitated all the males in his community for several days, thus leaving his home and possessions with no protection at all (save God!). — Morris, pages 335-336.
Wechsler takes a different view.
After establishing the rite of circumcision God changes Sarai’s name to Sarah, just as He earlier changed Abram’s to Abraham (v.5), further underscoring His active role as the Covenant Maker versus the “passive” role of His chosen covenantee(s). Following this he once again affirms His promise to give Abraham a son by her, in response to which Abraham fell on his face and—rather then worshiping God, as He does in v.3—laughed, asking instead that Ishmael be confirmed as his heir and inheritor of the Promise. To this God responds—as we’ve now come to expect—with gentle patient unflappable grace (a perfect Father!), declaring that it will not be Ishmael with whom He will maintain (which is preferable here to “establish,”) His covenant, but rather the son whom Sarah will bear to Abraham the following year. God also tells Abraham to name that son Isaac—meaning “he laughs”—thus serving as a continual reminder to the parents, from the moment of Isaac’s birth, of their faithless response (Sarah laughs also in Genesis 18:13) to the promise that God nonetheless faithfully fulfilled (the negative nuance of the laughter is also given a positive twist per Genesis 21:6). —Wechsler, page 196.
So which is it? Did Abraham respond to God in doubt or in faith. On my first reading of the passage, my view was that of Wechsler, but I also think Williams and Morris make some good points. So … I’m not sure. I think I’m still inclined toward Wechsler’s view based on a straightforward reading of the passage and since the Bible explains how Sarah was later rebuked for laughing and no explanation is given regarding how her response was different.