The term antichrist is only used four times in Holy Scripture:
“Little children, it is the last hour; and as you have heard that Antichrist cometh, even now there are become many Antichrists: whereby we know that it is the last hour.” (I John 2:18)
“Who is a liar, but he who denieth that Jesus is the Christ? This is Antichrist, who denieth the Father, and the Son” (I John 2:22)
“And every spirit that dissolveth Jesus, is not of God: and this is Antichrist, of whom you have heard that he cometh, and he is now already in the world.” (I John 4:3)
“For many seducers are gone out into the world, who confess not that Jesus Christ is come in the flesh: this is a seducer and an antichrist.” (II John 1:7)
Pope St. Pius X declared in E Supremi, (On the Restoration of All Things in Christ), Oct. 4, 1903:
5. When all this is considered there is good reason to fear lest this great perversity may be as it were a foretaste, and perhaps the beginning of those evils which are reserved for the last days; and that there may be already in the world the “Son of Perdition” of whom the Apostle speaks (II. Thess. ii., 3). Such, in truth, is the audacity and the wrath employed everywhere in persecuting religion, in combating the dogmas of the faith, in brazen effort to uproot and destroy all relations between man and the Divinity! While, on the other hand, and this according to the same apostle is the distinguishing mark of Antichrist, man has with infinite temerity put himself in the place of God, raising himself above all that is called God; in such wise that although he cannot utterly extinguish in himself all knowledge of God, he has contemned God’s majesty and, as it were, made of the universe a temple wherein he himself is to be adored. “He sitteth in the temple of God, showing himself as if he were God” (II. Thess. Ii., 2).
Speray's Catholicism in a Nutshell 