Jamieson, Fausset and Brown on Mark 13:6

6. For many shall come in my name, saying, I am Christ--(see Mt 24:5)--"and the time draweth nigh" (Lu 21:8); that is, the time of the kingdom in its full splendor.

and shall deceive many--"Go ye not therefore after them" (Lu 21:8). The reference here seems not to be to pretended Messiahs, deceiving those who rejected the claims of Jesus, of whom indeed there were plenty--for our Lord is addressing His own genuine disciples--but to persons pretending to be Jesus Himself, returned in glory to take possession of His kingdom. This gives peculiar force to the words, "Go ye not therefore after them."