The Catechism of the Summa - Tertia Pars - VI. OF THE GRACE OF CHRIST IN SO FAR AS HE IS HEAD OF THE CHURCH (B)

(B)
Is there a head in the order of evil that leads men to their loss just as Christ in the order of good leads men to salvation?

Yes; and this head of the wicked is Satan, the Prince of the devils (VIII. 7).

In what sense is Satan the head of the wicked?

Not in the sense that he can communicate evil to man interiorly, as Christ communicates good; but in the order of external government he strives to turn men away from God, whereas Christ leads men to God; and all those who sin imitate his rebellion and his pride, whereas the good by their works imitate the submission and obedience of Jesus Christ (VIII. 7).

Is there then on account of this opposition as it were a personal struggle between Christ and Satan?

Yes.

What will be the end of this struggle?

This struggle will rise to such a pitch that Satan will concentrate the whole of his power and malice in some individual human being who will be called Antichrist.

Will Antichrist in a special way be the head of the wicked?

Yes, for there will be more malice in him than there ever was in any other man; he will be Satan's vicar, whose object will be to strive his utmost in order to lead men to damnation and so ruin the Kingdom of Jesus Christ (VIII. 8).

Next - The Catechism of the Summa - Tertia Pars - VII. OF THE KNOWLEDGE OF CHRIST: OF HIS BEATIFIC KNOWLEDGE; OF HIS INFUSED KNOWLEDGE; AND OF HIS ACQUIRED KNOWLEDGE (A)