only the Holy Spirit

Reason and free will are able to a certain extent to live an outwardly decent life; but to be born anew, and to obtain inwardly another heart, mind, and disposition, this only the Holy Ghost effects. He opens the understanding and heart to understand the Scriptures and to give heed to the Word, as it is written Luke 24:45: Then opened He their understanding that they might understand the Scriptures. Also Acts 16:14: Lydia heard us; whose heart the Lord opened that she attended unto the things which were spoken of Paul. He worketh in us both to will and to do of His own good pleasure, Phil. 2:13. He gives repentance, Acts 5:31; 2 Tim. 2:25. He works faith, Phil. 1:29: For unto you it is given, in behalf of Christ, not only to believe on Him. Eph. 2:8: It is the gift of God. John 6:29: This is the work of God, that ye believe on Him whom He hath sent.
Solid Declaration, FC 1580