what does papa tell liesel when he sees she has stolen a book from the fire?

Liesel shows Hans the book she stole from the book burning, The Shoulder Shrug. Hans examines it and becomes worried about its content and the political repercussions of Liesel's act.

"This is our secret, this book. We will read it at night or in the basement, just like the others- but you have to promise me something."