I. Christ's arrest fulfils His own eternal purpose
Having discoursed, dearly beloved, in our last sermon, on the events which preceded the Lord's arrest, it now remains, by the help of God's grace, to discuss, as we promised, the details of the Passion itself. When the Lord had made it clear by the words of His sacred prayer that the Divine and the Human Nature was most truly and fully present in Him, showing that the unwillingness to suffer proceeded from the one, and from the other the determination to suffer by the expulsion of all frail fears and the strengthening of His lofty power, then did He return to His eternal purpose, and “in the form of a” sinless “slave” encounter the devil who was savagely attacking Him by the hands of the Jews: that He in Whom alone was all men's nature without fault, might undertake the cause of all.
The sins of darkness, therefore, assailed the true Light, and, for all their torches and lanterns, could not escape the night of their own unbelief, because they did not recognize the Fount of Light. They arrest Him, and He is ready to be seized; they lead Him away, and He is willing to be led; for though, if He had willed to resist, their wicked hands could have done Him no harm, yet thereby the world's redemption would have been impeded, and He, who was to die for all men's salvation, would have saved none at all.
Source: Sermons (New Advent)