Mephisto: Look, the world fell into chaos becaus I failed. We must do something to counter this.
Mephisto: You must go.
Rin: Where?
Mephisto: To the past.
Mephisto: You already have it.
Rin:

Mephisto: The key to the past. The one and only Master Key that can lead tho any dimention it's owner wants.
Rin: That!? Even though I had it all this time (I never used it)! You should have told me sooner!!
Mephisto: It wasn't the right time (to tell you) until now.
Mephisto: Eins, zwei, drei.
Mephisto: This is a parting gift. If you put on the hood, it has the same effect as the camouflage poncho.
Mephisto: You must also hide that heart. I'll do it for you.
Mephisto: Any door is fine. You can use that one. [explains how to use the key]
Rin: And you?
Mephisto: Oh? What's wrong? Are you scared? [laughs]
Rin: I am scared. The truth is, if I could, I wouldn't want to know. And to be honest, if you're gonna show this to me so readily, I would have rather had you show this to Yukio instead... He wanted to know.
Mephisto: Humans avert their eyes from reality. They have eyes to see, but they are lost in the fog. In truth, this is comical/funny/humorous.
Mephisto: If you want to move forward, you must look at reality as it is.
Mephisto: The reality Okumura-sensei has to see, unfortunately, isn't in the past.
Mephisto: But, yours is in the past.
Mephisto: And most importantly, I have a predilection not to give people what they want and to give them what they do not want.
Rin: You're the worst.
Mephisto: Of course. I'm a demon after all. Now go.