"Whom the Lord Jesus will slay," depriving him of life, striking him dead as a mortal man worthy of every punishment. "With the spirit of His mouth," that is, either by word or by breath. He will speak a word, or merely breathe, and that one's life will be cut short.
According to Oecumenius, the spirit of the mouth is a command and order, and as it were a breath. Isaiah had already foreseen that "the rod from the root of Jesse... will slay the wicked one with the spirit of His lips" (Is 11:1, 4).
In the Apocalypse, Christ the Lord, named the Word of God, is depicted in such a way that a sharp weapon comes forth from His mouth. This is evidently a sign of the decisiveness, swiftness, and unsparing character of God's righteous judgment and the wrath accompanying it.
As a result, He will cast both the beast and his false prophet, who deceived many by his signs, into the lake of fire burning with brimstone (Rev 19:15, 20).
Therefore one may suppose that in the Apostle also, slaying with the spirit of His mouth is the final decision of the lawless one's fate by his overthrow into Hades.
"And will destroy him," making his place empty and excluding him from the number of the living. This is the same as "will slay." The Greek word also hints that, by destroying the lawless one, the Lord will ruin and undo his works.
All his pomp and all his deceptive illusions will disappear. Everyone will see his nothingness and the falsehood of his display.
"By the manifestation of His coming," that is, by the act of His coming, by the coming itself, or by the manner of His coming, by the form in which it takes place. Both meanings indicate this.
As wax melts before fire, and smoke vanishes before the face of the wind, so the lawless one and all his work will not stand before the Lord coming to judgment. Everything will be scattered like dust.
So majestic, almighty, and fearful for the man of lawlessness and all sinners will be the coming of the Lord.
Blessed Theodoret writes: "The Apostle showed, as far as possible, the greatness of the Master's power. For he says that, having come from heaven, He will only speak and will deliver the destroyer to final perdition.
"The Prophet Isaiah also foretold this. Having said, 'A rod shall come forth from the root of Jesse,' and having described what this rod is, he added, 'and with the spirit of His lips He shall slay the wicked one'" (Is 11:1, 4).
It seems, however, that in this depiction one may also see the all-purifying power of the Lord's second coming "in flaming fire," as was mentioned above (2 Thess 1:8).
Now every sin and lawlessness walk through the streets of the world with head raised, mixed with virtue and righteousness, and even prevailing over them. Then the righteous will be separated from sinners, and the latter will be enclosed in one place from which there is no departure.
Only purity, holiness, and righteousness will remain on the world's stage. The Apostle depicts this coming purification of all that exists under the image of the destruction of the lawless one and his work by the manifestation of the Lord's coming.
The Lord will come. As light drives away darkness, and wind drives away mist, so He will drive away and confine every unrighteousness and sin to the place appointed for them.
Then the all-radiant kingdom of truth and goodness will come.
The Apostle speaks of the nothingness of the lawless one and all his illusory pomp before the majesty and power of the Lord before depicting his flattering and dangerous snares against our weakness. He does this to keep hope and trust in the Lord watchful and firm.
He also does this so that his later depiction of the antichrist's snares will not bring despondency and terror upon the Thessalonians. He impresses this upon them: the son of perdition will come, but do not fear. See what kind of Lord you believe in.
Saint Chrysostom writes: "The lawless one will appear. What then? Immediately after this the Apostle gives consolation, for he adds, 'whom the Lord will slay.'
"Just as fire, when it merely approaches, even before it comes near, paralyzes and destroys small creatures that are still far off, so Christ will slay the antichrist by His mere appearing and coming.
"It is enough that He appear, and all this will perish. As soon as the Lord appears, He will bring deception to an end."