Apocalypse: All is revealed.
Apocalypse: Everything they've built will fall! And from the ashes of their world, we'll build a better one.
Erik Lensherr: Does it ever wake you in the middle of the night? The feeling that one day, they'll come for you? And your children?
Charles Xavier: I feel a great swell of pity for the poor soul who comes to my school looking for trouble.
Apocalypse: You are all my children, and you're lost because you follow blind leaders. No more false gods. I'm here now.
Erik Lensherr: [sees Apocalypse for the first time.] Who the fuck are you?
Answer: As shown in Days of Future past, Trask was able to identify Magneto and Mystique, as well as their mutant powers. Trask could have found Charles Xavier at any given time, or Stryker possibly researched him. Stryker did work for Trask after all.