To search for text in the syntax editor, press CTRL-I.
The syntax editor performs an "incremental search", which is different from the kind of text search that you are probably used to.
After you press CTRL-I, the mouse pointer changes to indicate that you are performing an incremental search. Start typing the string that you want to search for. Say you start to type "authoritarianism". As soon as you type the character "a", the syntax editor highlights the first occurrence of "a" that it finds. Next type "u", and the syntax editor highlights the first occurrence of "au" that it finds. And so on.
After you have found one occurrence of the text you are searching for, press CTRL-I again to search for the next occurrence.
If "authoritarianism" is present in the text, there is a good chance that you will find it before typing the entire word.
TIP: It can be useful to specify highlighting for matching strings before performing an incremental search.