“Regeneration is monergistic: that is, entirely the work of God the Holy Spirit. It raises the elect among the spiritually dead to new life in Christ (Eph. 2:1-10). Regeneration is a transition from spiritual death to spiritual life, and conscious, intentional, active faith in Christ is its immediate fruit, not its immediate cause.” (J.I. Packer)
For more on the subject, visit the Systematic Theology page.