Salvation is a work that God does.  The success of His Gospel and the fruits from His ministry is not based upon how his ministers perform.  He has chosen to work in and through the foolishness of our words and the flawed efforts of our hands and feet to minister.  So for salvation to happen—for ministry to be fruitful, God has to be in it.  He has to do a work.  You could deliver a message with the intelligence of R.C. Sproul and the delivery of John MacArthur and the passion of John Piper and if God is not in it, nothing fruitful will come from it. 

God calls—He draws—He changes us from the inside out.  He sent His Son to accomplish salvation for us, He sends His people to share Christ with us, and He uses His word to instruct us and sends His Spirit to convict and convert us. 

There are so many places we find this in scripture.  Paul says in Ephesians 2:8-9,

For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.   

We are saved by God’s Grace, not by our works.  And though he says that we are saved “through faith,” Paul follows that statement with, “…this is not your own doing.”  We can’t even say our faith is from us because God is the one who awakens us to faith.  He also says in Titus 3:5,  

(God) saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit.

Jesus also said in John 14:17, “The world cannot receive (The Holy Spirit).”  Why?  He says, “It neither sees Him nor knows Him.”  We are spiritually dead as Paul says in Ephesians 2 and there is only one who can bring a spiritually dead person to life and that’s God.