Predestination (Ephesians 1:1-14)

God is sovereign in salvation. He chooses who He will save. Jesus said: "You did not choose me, I chose you". Ephesians 1 says: "he chose us in him, before the foundation of the world...He predestined us to be adopted...we were predestined according to the plan of the one who works out everything in agreement with the purpose of his will". But this raises some big questions: Why does God choose some and not others? What about free will? Why bother sharing the gospel if God has predetermined who will be saved? How do I know if He's chosen me? How should predestination impact the way I relate to God?