Sarah appears to have so confidently trusted this verse in Isaiah that says, "When the time is right, I, the Lord, will make it happen" (Isa. 60: 22b). She judged God faithful and wouldn't think otherwise. When we encounter delays or misfortune, we often attempt to accuse God, but that's a trick from the pit of hell. Scriptures bear eloquent testimonies that only good and perfect gifts come from the Lord (James 1: 17); no evil proceeds from God. Where then do people find what to accuse God with? Has He been faithful to you?
When hard times hit David, he remembered the faithfulness of God and challenged his circumstances with this question, "Do you know who the Lord is to me?" It inspired Psalm 23, "The Lord is my Shepherd." He was telling his circumstances that God is to him what he was to the sheep in the wilderness: he led them to find pasture and refreshing water; he defended them against the wild animals and ensured they were protected. Although faced with many battles, David kept praising and eulogising God for his faithfulness. Would you have judged God faithful if you were David? How about Job's response to the calamity that befell him?
God's faithfulness is not predicated on what he's done or failed to do but on His steadfast and consistent nature.