The first question of the Heidelberg Catechism views the atoning work of Christ as dealing with the satisfaction made for all our sins (penal substitution) and his redeeming us from all the power of the devil (Christus Victor).
What is your only comfort in life and in death?Thus the Catechism holds together what ought never to be separated. Here we have the Godward dimension of the atonement (satisfaction) and the polemic dimension (conquest). The latter, however, is dependent on the former.
That I, with body and soul, both in life and in death, am not my own, but belong to my faithful Savior Jesus Christ, who with His precious blood has fully satisfied for all my sins, and redeemed me from all the power of the devil; and so preserves me that without the will of my Father in heaven not a hair can fall from my head; indeed, that all things must work together for my salvation. Wherefore, by His Holy Spirit, He also assures me of eternal life, and makes me heartily willing and ready from now on to live unto Him.
When Scripture explicates how Christ conquers the devil, the reality of which is anticipated in the proto-evangelium (Gen. 3:15), it views the power of the devil as the power of deception and accusation. Our legal position before God, in view of Adam's breaking of the covenant of works (Gen. 2:15-17), and our own sins, has rendered us guilty, cursed, and under the sentence of death (Rom. 6:23).
How does Christ redeem us from the power of the devil?
By dying for us (1 Peter 3:18). By taking our curse and punishment (Gal. 3:13). By enduring the wrath of God (Rom. 3:25-26). By taking the full penalty of the law (Gal. 3:10).
The legal accusations of Satan are silenced by the blood of the Lamb that has brought us forgiveness for all our sins (Col. 2:13-15; Eph. 1:7; Rev. 12:10-11; Rom. 8:1, 33-34!).
How has Christ conquered Satan?
By his active and passive obedience, by making atonement and justification. And now without God's law to condemn us, Satan has no power to accuse us (1 Cor. 15:56). What truth then will he seek to overthrow with all his might? The truth that the blood of the Lamb saves, the doctrine of penal substitution.
The Lamb slain saves us. The Lamb slain silences Satan's accusations. It is seeing this connection that will stop the pendulum from swinging from penal substitution to Christus Victor. As Henri Blocher argued, in a much neglected essay, these doctrines are seen in the biblical proportions and glory together. It is really Agnus Victor, not what is commonly understood as Christus Victor, that best explains the conquering of Satan.