Just as simplicity and truth are separate entities, so are extremism and greatness separate entities: Extremism is the perfection of the topic. One who waves the banner of moderation and hates extremism, is in the same camp as liars or imbeciles. If there is no extremism there is no perfection and without perfection there is no beginning. For the beginning is with constant questioning and doubts, and perfection is the sharp reply which puts each statement in the right and truthful place.
We are used to hearing certain circles declare that they are not extremists, and yet still consider themselves faithful Jews with enough faith in Torah and Torah opinion. From an arbitrary point of view, we say that just as there are among the lovers of wisdom none who love just a little wisdom and hate a lot of wisdom, so among the lovers of Torah and its commandments there is no love of mediocrity and hatred for extremism.
The above is a quote from the Chazon Ish, Yalkeit Daas Torah.