timdiggerm said:

This is one of the big mysteries of the film. Why is Luke in seclusion? Why hasn't he trained anyone?

My take on it is that he has trained some Jedi before going into hiding. Maz Kanata for one, as she seems to have force training of some sort judging by her dialogue. Where would she have got that from if not from Luke?

Also Han says "They're real" referring to the Jedi. Not they were real, but they ARE. My guess is that there are quite a few former pupils of Luke's scattered about the galaxy, having gone into seclusion because of...something.