Objection 1. It would seem that Anti-christ is not the head of the wicked. For there are not several heads of one body. But the devil is the head of the multitude of the wicked. Therefore Anti-christ is not their head.
Objection 2. Further, Anti-christ is a member of the devil. Now the head is distinguished from the members. Therefore Anti-christ is not the head of the wicked.
Objection 3. Further, the head has an influence over the members. But Anti-christ has no influence over the wicked who have preceded him. Therefore Anti-christ is not the head of the wicked.
On the contrary, A gloss [St. Gregory, Moral. xv] on Job 21:29, "Ask any of them that go by the way," says: "Whilst he was speaking of the body of all the wicked, suddenly he turned his speech to Anti-christ the head of all evil-doers."
I answer that, As was said above (Article 1), in the head are found three things: order, perfection, and the power of influencing. But as regards the order of the body, Anti-christ is not said to be the head of the wicked as if his sin had preceded, as the sin of the devil preceded. So likewise he is not called the head of the wicked from the power of influencing, although he will pervert some in his day by exterior persuasion; nevertheless those who were before him were not beguiled into wickedness by him nor have imitated his wickedness. Hence he cannot be called the head of all the wicked in this way, but of some. Therefore it remains to be said that he is the head of all the wicked by reason of the perfection of his wickedness. Hence, on 2 Thessalonians 2:4, "Showing himself as if he were God," a gloss says: "As in Christ dwelt the fulness of the Godhead, so in Anti-christ the fulness of all wickedness." Not indeed as if his humanity were assumed by the devil into unity of person, as the humanity of Christ by the Son of God; but that the devil by suggestion infuses his wickedness more copiously into him than into all others. And in this way all the wicked who have gone before are signs of Anti-christ, according to 2 Thessalonians 2:7, "For the mystery of iniquity already worketh."
Reply to Objection 1. The devil and Anti-christ are not two heads, but one; since Anti-christ is called the head, inasmuch as the wickedness of the devil is most fully impressed on him. Hence, on 2 Thessalonians 2:4, "Showing himself as if he were God," a gloss says: "The head of all the wicked, namely the devil, who is king over all the children of pride will be in him." Now he is said to be in him not by personal union, nor by indwelling, since "the Trinity alone dwells in the mind" (as is said De Eccles. Dogm. lxxxiii), but by the effect of wickedness.
Reply to Objection 2. As the head of Christ is God, and yet He is the Head of the Church, as was said above (1, ad 2), so likewise Anti-christ is a member of the devil and yet is head of the wicked.
Reply to Objection 3. Anti-christ is said to be the head of all the wicked not by a likeness of influence, but by a likeness of perfection. For in him the devil, as it were, brings his wickedness to a head, in the same way that anyone is said to bring his purpose to a head when he executes it.
Source: Summa Theologica (New Advent)