Regeneration is the divine work of God the Holy Spirit upon the minds and souls of fallen people

However, when we consider the teaching on this issue as found in John’s record of Jesus’ discussion with Nicodemus, we see the emphasis that Jesus places on regeneration as a necessary condition, a sine qua non, for believing in Him. He says to Nicodemus in John 3:3: “Truly, truly, I say to you, unless one is born again, he cannot see the kingdom of God.” Then again in verses 5–7, Jesus says, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, ‘You must be born again.’” The must-ness of regeneration of which Jesus speaks is necessary for a person to see even the kingdom of God, let alone to enter it. We cannot exercise faith in a kingdom that we cannot enter apart from rebirth. 

The weakness of all semi-Pelagianism is that it invests in the fallen, corrupt flesh of man the power to exercise faith. Here, fallen man is able to come to Christ without regeneration, that is, before regeneration. On the other hand, the axiom that regeneration precedes faith gets to the very heart of the historic issue between Augustinianism and semi-Pelagianism. 

In the Augustinian and Reformation view, regeneration is seen first of all as a supernatural work of God. Regeneration is the divine work of God the Holy Spirit upon the minds and souls of fallen people, by which the Spirit quickens those who are spiritually dead and makes them spiritually alive. This supernatural work rescues that person from his bondage to sin and his moral inability to incline himself towards the things of God. Regeneration, by being a supernatural work, is obviously a work that cannot be accomplished by natural man on his own. If it were a natural work, it would not require the intervention of God the Holy Spirit. 

RC Sproul
- The New Birth