From his introduction:
התשובה היא תופסת את החלק היותר גדול בתורה ובחיים, עליה בנויות כל התקוות האישיות והציבוריותTeshuvah [repentance] occupies the greatest portion of Torah and Life, and upon it are founded all of the hopes of the individual and the community.
From Perek 5:
העולם מוכרח הוא לבא לידי תשובה שלימהThe universe is compelled to come to complete teshuvah.
And, most powerfully, from Perek 6:
אין דבר בטוח בעולם כמו התשובהNothing in this universe is as certain as teshuvah.
I cry when I read this sentences. Okay, I cry easily, but still - I haven't been this moved by a Torah text since the first time I read Rav Shimshon Raphael Hirsch.
Rav Kook spends a lot of time on the mystical character of teshuvah - how it ties into the fundamental nature of the universe, why it is necessary, how it influences one's actions in the past as well as the future - but he doesn't use mystical jargon. I find it inspiring, very readable, and very worthwhile.
I'm embarrassed it's taken me this long to learn it כסדר.