If your set of beliefs implies that your set of beliefs are not true, that's a contradiction.
In this case, thinking all of reality is material implies that you cannot believe your thoughts, including the thought that all of reality is material. But that was your original belief, meaning through your own worldview you've reached a contradiction.
In this case, thinking all of reality is material implies that you cannot believe your thoughts, including the thought that all of reality is material. But that was your original belief, meaning through your own worldview you've reached a contradiction.