A reaction statement allows you to respond in your own words to some important point or theme in a work that has appealed to you.  Good books always "speak" to you; so when you have read a good book, you will always have something significant to say about it in the form of a personal commentary.  Let it be about one page worth of matter. 

Another way of approaching this is to answer the question, for example, "What did St Augustine" tell you?  Your answer to this question will be a good enough reaction, provided you draw the needed support from the book.