False Start!

All runners in a race line up at the "starting line."  No one is allowed to cross that line until the starter's gun is fired..  It's called a "false start" if they cross that line ahead of the gun.  Well, that's exactly what Sarah did....she got ahead of God's starting time....she had a "false start!"

Abraham "did not waver at the promise of God through unbelief, but was strengthened in faith..".  In other words, he didn't doubt God's promise that they were to give birth to a son.  Sarah was going to help God to hurry up the process.  She "conceived" a plan of her own:  She offered her handmaiden to Abraham.  Her plan worked....they had a son, named Ismael. 

Both disobeyed God!  Abraham gave into Sarah's plan rather than waiting for God's.  Sarah was impatient; Abraham maybe welcomed the pleasure of sleeping with another woman....or maybe he wanted peace (at all costs) in his household, as he was weary of hearing Sarah's complaints about waiting.

Of course, in God's timing, Isaac was born--they were blessed with the child that God promised them.  However, the child of their own making,Ismael, caused them much trouble because they had  "jumped the gun" and rushed ahead of God's timing.

Sarah grew weary of waiting....it weakened her faith.  But remember--"those who wait upon the Lord shall renew their strength...".  That is, if they wait "in faith."  [See:  Isaiah 40:31 and Romans 10:17]  Satan always causes us to rush ahead of God.  Don't have a "false start"...WAIT for His perfect timing.

REASONING (leading with your mind) rushes ahead of God... FAITH (believing with your heart) follows God!