Ephesians 1:9

"He made known to us the mystery of His will, according to His good pleasure that He planned in Him."

     The Greek word is mysterion. It literally means "a secret." So what's he talking about? Well, Barclay, the scholar, said that a mystery is hidden from the heathens, but clear to the Christian. So in other words, what happened? God had determined, and he said this, in Genesis 3, to the serpent that just deceived Adam and Eve

Genesis 3:15

"I will make you and the woman enemies to each other. Your descendants and her descendants will be enemies. Her child will crush your head. And you will bite his heel."

     What was God doing? Even in Genesis 3, as soon as we had sinned against Him, He said, "Hey, I've got a plan. And the plan is that there's a day coming when my son will crush the head of the serpent." Now, Satan didn't understand that. Why? Because it was a mystery. GOD HID IT. Why did he hide it? So that satan and all of his minions would not know God's plan

Because God doesn't reveal His plan to the wicked. 

He reveals His plan to the divine counsel, and to His people. 

     And so here we have this mystery hidden. So throughout all of the Old Testament, God is working through Israel, God is working through the prophets, God is working until in the fullness of time, God is fulfilling his plan that God Himself will leave heaven, come to earth, take on flesh, walk upon the earth for 30 years, minister for 3 years. And then He will be lied about, deceived, attacked, He will be taken down, beaten and nailed to a cross. 

What a mystery that the God who is eternal would die-

That the God who is infinite would take on flesh and that God would die.

And Satan thought when he crucified Jesus, because he didn't know the mystery, he thought, "I HAVE ONE. I've killed the Son of God." He did not know that God was using even Satan, and the wicked religious leaders to accomplish His will. And that they crucified Jesus so that He could die for you and me. Nailed him to a cross. And finally He said, 

IT IS FINISHED

Tetelestai! 

    He had done everything necessary to save us. He was put in a borrowed tomb, and on the third day He rose from the dead. And that is the mystery that you and I know. There is no other name given by which you can be saved but Jesus.  Jesus, Jesus, Jesus. That's the mystery He's made known to us. Do you ever wonder why the whole world out there doesn't see it? Because the God of this world has blinded their mind. It is hidden to the heathen, but it is clear to the Christian.