Q: How are we made partakers of the redemption purchased by Christ?
A: We are made partakers of the redemption purchased by Christ, by the effectual application of it to us by his Holy Spirit.

Q: How doth the Spirit apply to us the redemption purchased by Christ?
A: The Spirit applieth to us the redemption purchased by Christ, by working faith in us, and thereby uniting us to Christ in our effectual calling.

Q: What is effectual calling?
A: 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.

(Questions XXIX, XXX and XXXI from the Westminster Shorter Catechism)