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.

John 1:12-13. But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
John 3:5-6. Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. . . That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
Titus 3:5-6. Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; which he shed on us abundantly through Jesus Christ our Saviour.

Westminster Project - See Francis Beattie on Union with Christ, Effectual Calling, and Regeneration. These are all topics covered in this section of the Catechism (Questions 29-32)