Abraham was 99 and Sarah 90 when God promised them a son. Any couple that has experienced infertility can relate to their pain and dashed hopes. But at this age, any of us would have been planning the end of our life, not dreaming of the unexpected gift of new life!
And yet it is into this situation of complete physical impossibility that God enters with His promise of a son, to which they respond with hardened cynicism. Who would blame them? But God loves to redeem such situations because only He can make something out of nothing, create life in barren wombs, and raise the dead to life again.
God was setting the stage for another impossible pregnancy, one conceived by the Holy Spirit to create a child born of the will of the heavenly Father and the seed of the woman. Jesus was fully man and fully God, the ultimate child of the promise who would see the results of His suffering and be satisfied. He would inaugurate for us the end of fruitlessness, meaninglessness, frustration, pain, and death. God will have the last laugh!