Q: How does the Spirit apply to us the redemption purchased by Christ?
A: The Spirit applies to us the redemption purchased by Christ, by working faith in us, and thereby uniting us to Christ in our effectual calling.
John 15:5: I am the vine, you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing.
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 does persuade and enable us to embrace Jesus Christ, freely offered to us in the gospel.
Ephesians 1:17-18: that the God of our Lord Jesus Christ, the Father of glory, may give you the Spirit of wisdom and of revelation in the knowledge of him, having the eyes of your hearts enlightened, that you may know what is the hope to which he has called you, what are the riches of his glorious inheritance in the saints.
These two questions really do belong together because they're answering two sides of the same question: How are sinful men saved?
The first thing the catechism wants to put before us is that it is by faith that we are united to Jesus Christ, and therefore benefit from the redemption that he has accomplished. By faith alone, by our acceptance, reliance, full trust in what Christ has done for us, in Christ's perfect obedience and righteousness, in Christ's life, death, resurrection, and future coming, are we united to him. And the key word here is "trust". Faith is not simply knowing and believing in Christ. As James would say, "even the demons believe"! Rather, faith is that trust that says, "no merit of my own, His anger to suppress, my only hope is found in Jesus' righteousness". Faith is a trust, it is a reliance upon the righteousness of Christ because true faith recognizes that we are absolutely wicked and condemned before a holy and righteous Judge.
Okay, fine. We are united to Christ by faith, but we have a problem! We spent all these weeks talking about the fall of man, man's depravity, man being in an estate of sin and misery, man being dead in sins and trespasses. So how can man possibly even have faith? Where does this faith come from? If we are enemies of God, if, in our natural state, we hate the things of God, if Paul is right in Romans 3 that, "no one understands; no one seeks for God", then how can we ever trust and rely upon the work of Jesus Christ? And this is the second aspect of salvation that the catechism is addressing in these two questions. Mankind comes to faith in Jesus Christ by effectual calling.
Effectual calling is part of God's eternal decrees. It's the Holy Spirit's work in regenerating those who the Father has elected to eternal salvation. It's the work of the Spirit in raising dead men to life, giving them the faith they need to call on Christ. This is how dead sinners have faith in Christ! They have faith because the Holy Spirit gives them new hearts! They have faith because faith itself is a gift from God! Notice the language here. The writers of the catechism want to be very careful to preserve man's free will. The Spirit convinces us of our sin and misery. The Spirit persuades and enables us to embrace Christ. But make no mistake about it. Those who the Father have called WILL be convinced of their sin and misery, and they WILL embrace Christ in faith. It is a guaranteed outcome. It is "effectual"!
All of this runs contrary to the thoughts of many concerning salvation. Often, many Christians teach and believe that first we have faith, and then we are regenerated and brought to new life in Jesus. But that order of salvation does not take into account how utterly wicked men are. It doesn't take our sin and depravity as seriously as it needs to be taken. It doesn't put much stock into the words of Paul in Romans 3, and therefore never addresses the question of, "how do dead, evil men come to faith in the first place?" The catechism though, and Scripture itself, teaches otherwise. Here we see, it is not our faith that leads to regeneration, but just the opposite. Brothers and sisters, it is because the Holy Spirit has raised us from the dead, because he has regenerated us, because he has given us new hearts that we are able to say, "Jesus Christ is Lord!" Even our faith is a gracious and merciful act from a gracious and merciful Father!