Mark 13:6

(Epist. cxcix. 9.) In answer to the disciples, the Lord tells them of things which were from that time forth to have their course; whether He meant the destruction of Jerusalem which occasioned their question, or His own coming through the Church, (in which He ever comes even unto the end, for we know that He comes in His own, when His members are born day by day,) or the end itself, in which He will appear to judge the quick and the dead.

(ubi sup.) For many came forward, when destruction was hanging over Jerusalem, saying that they were Christs, and that the time of freedom was now approaching. Many teachers of heresy also arose in the Church even in the time of the Apostles; and many Antichrists came in the name of Christ, the first of whom was Simon Magus, to whom the Samaritans, as we read in the Acts of the Apostles, listened, saying, This man is the great power of God: wherefore also it is added here, And shall deceive many. (Acts 8:10)

But before answering their question, He strengthens their minds that they may not be deceived, wherefore there follows: And Jesus answering them began to say, Take heed lest any man deceive you? And this He says, because when the sufferings of the Jews began, some arose professing to be teachers, wherefore there follows: For many shall come in my name, saying, I am Christ; and shall deceive many.