What is effectual calling?

ANSWER: Effectual calling is the work of God's Spirit, whereby, convincing us of our sin and misery, enlightening our minds in the knowledge of Christ, and renewing our wills, he doth persuade and enable us to embrace Jesus Christ, freely offered to us in the gospel.

--q.31 of Westminster Shorter Catechism

Scriptures that influenced this answer:
Acts 26:18. To open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me. 1 Corinthians 2:10, 12. But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.... Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God. 2 Corinthians 4:6. For God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ. Ephesians 1:17-18. That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him: The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints