From the book:

Indeed, calling to Allāh, enjoining good, and forbidding evil is a great foundational principle of Islām. Some scholars have even considered it among the pillars of Islām upon which its structure does not stand without. And this is not surprising, for the well-being of the servants in their livelihood depends upon obedience to Allāh and His Messenger, and perfecting that obedience is dependent upon enjoining good and forbidding evil. By way of this, this Ummah has become the best nation brought forth for mankind.
Due to the lofty position of this principle in the religion, and because most people in this era have become negligent regarding it — many abandoning it entirely, while others show undue leniency — the result has been the widespread emergence and dominance of evil, to the point that land, sea, and sky have become filled with it.
Thus, I saw it suitable to write a brief treatise on this matter, in which I clarify the great importance of this obligation, the consequences of neglecting it, and the discussions that follow from it, despite my limited capacity in doing so, hoping to fulfill at least part of the duty. Even though I am not among the people of that path.