Judgment is the sanction of law. Since Jesus came not to destroy the law but to fill it full of meaning, His commission to the Paraclete must effect the same purpose to honor the law by declaring its sentence against sin. Says Tholuck:
The meaning of our Lord's words is, When the divine principle of the Spirit shall spread among my disciples and produce its extraordinary effects in mankind, people will be forced to confess that the power of the evil spirit which opposes me in the ungodly feelings of men is broken. By the incarnation and coming of the Savior an inward judgment was commenced in the hearts of men of which the last judgment is only the outward manifestation.
The atoning death of Christ declaring God's abhorrence of sin and His mercy to sinners was the defeat of Satan, the usurping ruler called "the god of this world." Christ Jesus through death conditionally emancipated every human soul from Satanic bondage, and thus "destroyed him who hath the power of death, that is, the devil."
But how can the casting out of "the prince of this world" demonstrate that every one who refuses to trust in Christ with true obedience and to seek justification through reliance on the atonement for sin made by Him will be found on the left hand of the Judge and hear his own condemnation pronounced in the last day? Our answer is this: All who take on the Satanic character, must expect the Satanic doom; all who bear the devil's image must share his destiny. So all who bear the likeness of Christ will share His glory. "After the while dilemma between sin and righteousness is clearly set forth, the Spirit finally announces the judgment of Satan in such a way that He not only comforts believers with the perfect comfort declared in Rom. viii. 33, 34, but also reproves unbelievers with that awful judgment, as a last sting, in their inmost hearts, Will ye then absolutely be and continue the devil's? Will ye be judged with him?" (Stier.) Satan is condemned now for our benefit if we yield to the Spirit's voice in our hearts and accept the righteousness which Christ provides and the Comforter inworks; or we abide with God's great adversary in the judgment if we continue in sin with the world. This third and last conviction of the Spirit clearly implies that in the estimation of the Spirit of truth the existence of the devil must needs belong to some fundamental article of saving truth without which we cannot correctly estimate the enormity of sin, the value of righteousness and the necessity of the atonement by which it was procured. In this third conviction the victory of righteousness over sin is completed. In this our salvation is infallibly secured if we but will it. The Spirit never coerces a free agent.
— The Gospel of the Comforter, Chapter 8.