Hebrews 12:12-13

Therefore, strengthen your feeble arms and weak knees. "Make level paths for your feet", so that the lame may not be disabled, but healed. (NIV)

I could go on and on, but I've run out of time. There are so many more— Gideon, Barak, Samson, Jephthah, David, Samuel, the prophets....Through acts of faith, they toppled kingdoms, made justice work, took the promises for themselves. They were protected from lions, fires, and sword thrusts, turned disadvantage to advantage, won battles, routed alien armies. Women received their loved ones back from the dead. There were those who, under torture, refused to give in and go free, preferring something better: resurrection. (The Message Hebrews 11:32-35)

Their weakness was turned into strength.  That is how they could know it was God, and not themselves.  He took their impossible, and made it reality.