Jeremiah 1:11-12 tells us that God says, “I am watching over my word to perform it.” It not as though God makes a promise and then forgets about it. Even if it takes thousands of years – which it did for His promises about Messiah to come true – He remembers and keeps His word.
For this reason, Abraham was right to trust God and be “fully assured that what He had promised He was able also to perform” (Romans 4:21), and Sarah was right to consider Him faithful who had promised” (Hebrews 11:11).
God does not make His promises lightly. We may be a vapor that appears for a while and vanishes away, but God has a reputation to maintain before the host of heaven. Thus He is not just faithful to keep His promises, but zealous to do so as well.
Scripture quotations taken from the NASB, unless otherwise noted.