We seek refuge from al-masih al-dajjal in every prayer. I would always wonder, given how much we know about this being from the Prophet (s), how could anyone, let alone any Muslim, be deceived about him or not recognize him for who he is?
The answer is the dajjal is an imposter. He comes pretending to be the masih, i.e., the Messiah that the Jews, Christians, and Muslims of the world are awaiting. He will come with all the superficial indications and appearances of being this Messiah, but only true believers will see him for who he really is.
The same dynamic is happening with ISIS. They have all the appearances and superficial characteristics of being true believers and fighting for deen, and many people, Muslim and non-Muslim, see those things and believe them, namely, that ISIS is an Islamic phenomenon, i.e., that they are inspired by Islamic tenets, when in reality they are just imposters and have nothing to do with Islam.