There's no evidence that "many" of the refugees that gave entered Germany in the last year are a security threat so why do you think that?
edit: I said economic migrants who lie about being refugees are a security threat, not the actual Syrian refugees.
http://www.telegraph.co.uk/news/2016/12/20/everything-know-s...
https://en.wikipedia.org/wiki/Murder_of_Maria_Ladenburger
https://en.wikipedia.org/wiki/New_Year's_Eve_sexual_assaults...