Matthew 24:5

For many will come. One of these was Simon Magus, who in the Acts (chap. viii. ver. 10.) is mentioned as calling himself the power of God; hence the apostle St. John (1 epistle ii. 18,) says, and as you have heard that Antichrist cometh, even now there are become many Antichrists. By Antichrists I understand heretics, who, under the name of Christ, teach doctrines different from Christ; neither is there any reason for us to be surprised, if many be seduced, since our Lord declares that many will be seduced. (St. Jerome) ... This alone will be sufficient for us to know the false doctrines taught by Antichrist, when they assure us that they are Christ; for we do not read in any part that Christ said so of himself. The miracles he performed, the doctrines he taught, and the virtues he on every occasion exhibited, were proofs sufficient to convince us that he was the Christ. There is need of the assistance of God to overcome the snares laid for us by hypocrisy. (Origen) --- Among these impostors were one Theodas, (Acts v. 36,) the impious Egyptian, (Acts xxi. 38,) Judas of Galilee, Menander, and several others who preceded the destruction of Jerusalem; but many more will precede the destruction of the world. This therefore is the first sign, the seduction of many souls from the true faith by heresies, and is common to both events. (Jansenius) --- See much more in Barradius, tom. iii. lib. 9, chap. 2, where he collects various illustrations from Josephus and profane authors. (Menochius)