In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Al-Allamah Salih Aala Ash-Shaikh, may Allah preserve him, said:

The call [i.e. subject matter] in this book is the greatest goal- the essence of the divine messages, the means to attaining honour for the one who holds fast to it, knows and calls to it. It is elevation for the one whom Allah wants to elevate and they are of varying levels in the sight of Allah. Due to this, we advise that this book be understood properly and repeated. One should not think that he has studied and finished it, rather it should be repeated twice, thrice or four times– repeating it again whenever he completes it.

This is because he will not be able to understand it properly and acquire a clear and firm [understanding], except by repeating it many times. He will find knowledge he did not know previously every time he repeats it. This requires that you exercise patience- having a good suspicion of Allah, asking Allah to grant you Tawfeeq [i.e. the ability to learn and act]; to be steadfastness upon this lofty and great matter, and that you do not turn your attention to other than it; rather the scholars give precedence and embark upon the call to this great matter, which is Tawhid – the right of Allah upon His slaves. [Paraphrased]

Fat’hul Masjid Sharh Kitab Tawhid Vol 1 Introduction