From the Shorter Catechism

Q. 30. 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. Eph. 1:13-14; John 6:37, 39; Eph. 2:8; Eph. 3:17; 1 Cor. 1:9.