Therefore Sarah {g} laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?

(g) For she believed the order of nature, rather than believing the promise of God.

Wesley's Notes

18:12 Sarah laughed within herself - It was not a laughter of faith, like Abraham's, Ge 17:17, but a laughter of doubting and distrust. The great objection which Sarah could not get over was her age. I am waxed old, and past child - bearing in a course of nature, especially having been hitherto barren, and which magnifies the difficulty, My lord is old also. Observe here, That Sarah calls Abraham her lord, and the Holy Ghost takes notice of it to her honour, and recommends it to the imitation of all Christian wives, 1Pe 3:6. Sarah obeyed Abraham calling him lord, in token of respect and subjection.

Jamieson-Fausset-Brown Bible Commentary

12. Therefore Sarah laughed within herself-Long delay seems to have weakened faith. Sarah treated the announcement as incredible, and when taxed with the silent sneer, she added falsehood to distrust. It was an aggravated offense (Ac 5:4), and nothing but grace saved her (Ro 9:18).

Matthew Henry's Concise Commentary

18:9-15 Where is Sarah thy wife? was asked. Note the answer, In the tent. Just at hand, in her proper place, occupied in her household concerns. There is nothing got by gadding. Those are most likely to receive comfort from God and his promises, who are in their proper place, and in the way of their duty, Lu 2:8. We are slow of heart to believe, and need line upon line to the same purport. The blessings others have from common providence, believers have from the Divine promise, which makes them very sweet, and very sure. The spiritual seed of Abraham owe their life, and joy, and hope, and all, to the promise. Sarah thinks this too good news to be true; she laughed, and therefore cannot as yet find in her heart to believe it. Sarah laughed. We might not have thought there was a difference between Sarah's laughter and Abraham's, ch. 17:17; but He who searches the heart, saw that the one sprung from unbelief, and the other from faith. She denied that she had laughed. One sin commonly brings in another, and it is not likely we shall strictly keep to truth, when we question the Divine truth. But whom the Lord loves he will rebuke, convict, silence, and bring to repentance, and if they sin before him.