The Revealed Mystery

Eph 3
The mystery was made known to me by revelation, as I have written briefly.

This mystery "was made known" to Paul, which is just another way of emphasizing the passive nature of the apostles calling from God. In other words, God and God alone chose Paul as His Chosen tool or instrument for service, and Paul could not attribute this divine choice to anything in himself as he was a persecutor, murderer and accomplice of murders done on the members of The Way (the early Church) prior to his conversion. This divine Election was due to nothing other than the good pleasure of Paul's Creator-God. Paul (like all sinners) require divine revelation to understand this mystery - The Holy Spirit had to open his eyes and instruct him, he never on his own could have come to an understanding of the ways of God in salvation apart from this special work of Grace. Being (like all men) naturally spiritually dead in his sin, he could never have understood either his desperate need of a Sovereign Savior or the redemptive work of Christ apart from divine Election and the opening of his heart which was/is/and always will be the work not of ourselves but the work of the Spirit

Titus 3 
But when the goodness and loving kindness of God our Savior appeared,  He 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, whom He poured out on us richly through Jesus Christ our Savior, so that being Justified by His Grace we might become heirs according to the hope of eternal life.

John 3
Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” Do not marvel that I said to you, ‘You must be born again.’  The wind (like the Holy Spirit) blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.”