Promises are good, but only if they can be fulfilled.  That is the problem with so much commercial trading: orders not completed, goods not perfect, money not paid on time.  But when God makes a promise, He keeps it. And often He arranges things so that it seems that even God could not do anything - and then He does!

When Abraham was 99 years old (Genesis 17:1) and Sarah, his wife, was 10 years younger (Genesis 17:17) - God enabled them to conceive.  Sarah was beyond her menopause, and she thought it was impossible to have a child (Genesis 18:12), yet God fulfilled His promise to give them a child.  Isaac would be the human ancestor of many people, including the Lord Jesus Christ, and all who believed in Him would receive God's blessing and salvation (John 3:16).

God's apparent delay in fulfilling His original promise (Genesis 12:2) until it was humanly impossible to have a child, proved that it was God who did it.  Abraham and Sarah were as good as dead, from a biological reproduction point of view (Romans 4:19), but God's grace gave their bodies new life and Isaac was the proof.  It was grace: neither Abraham nor Sarah deserved God's intervention, but God did it for His own glory.

No true believer can ever say to God, ‘You must be good to me because I deserve it’!  No, but God's grace gives us what we do not deserve so that our lives might bring glory to Him.  Spiritually dead people are born again by His grace alone.  Prayers are answered by His grace alone.  And often when it seems that it is impossible for God to intervene, He acts!  Faith in Jesus is not a religious anti-depressant, but it tells God that we are ready to receive His grace.  Believing in Him is the start of a great adventure in which He is in control, His promises will be fulfilled and His glory will be displayed ... and we receive His blessing.  Yes, it is all the more amazing because we do not deserve it at all - but that is the whole point of grace.  Our part is simply to believe what God promises and cooperate with Him.  That is real faith (James 2:23-24).