Luke 21:8

(Orat. 1. cont. Arian.) For since we have received, delivered unto us by God, graces and doctrines which are above man, (as, for example, the rule of a heavenly life, power against evil spirits, the adoption and the knowledge of the Father and the Word, the gift of the Holy Spirit,) our adversary the devil goeth about seeking to steal from us the seed of the word which has been sown. But the Lord, shutting up in us His teaching as His own precious gift, warns us, lest we be deceived. And one very great gift He gives us, the word of God, that not only we be not led away by what appears, but even if there is ought lying concealed, by the grace of God we may discern it. For seeing that the devil is the hateful inventor of evil, what he himself is he conceals, but craftily assumes a name desirable to all; just as if a man wishing to get into his power some children not His own, should in the absence of the parents counterfeit their looks, and lead away the children who were longing for them. In every heresy then the devil says in disguise, "I am Christ, and with me there is truth." And so it follows, For many shall come in my name, saying, I am Christ; and the time draweth near.

Or perhaps He docs not speak of false Christs coming before the end of the world, but of those who existed in the Apostles' time.

For there were many leaders when the destruction of Jerusalem was at hand, who declared themselves to be Christ, and that the time of deliverance was drawing nigh. Many heresiarchs also in the Church have preached that the day of the Lord is at hand, whom the Apostles condemn. (2 Thess. 2:2.) Many Antichrists also came in Christ's name, of whom the first was Simon Magus, who said, This man is the great power of God. (Acts 8:10.)

Matthew adds a third question, that both the time of the destruction of the temple, and the sign of His coming, and the end of the world, might be inquired into by the disciples. But our Lord being asked when the destruction of the temple should be, and what the sign of His coming, instructs them as to the signs, but does not mind to inform them as to the time. It follows, Take heed that ye be not deceived.

Now His disciples did not at all perceive the force of His words, but supposed they were spoken of the end of the world. Therefore asked they Him, saying, Master, but when shall these things be? and what sign, &c. For before His descent from heaven, there shall come some to whom we must not give place. For the Only-begotten Son of God, when He came to save the world, wished to be in secret, that He might bear the cross for us. But His second coming shall not be in secret, but terrible and open. For He shall descend in the glory of God the Father, with the Angels attending Him, to judge the world in righteousness. Therefore He concludes, Go ye not therefore after them.