"Then I will come to them from before them and behind them, from their right and from their left, and You will not find most of them as thankful ones (i.e. they will not be dutiful to You)." (Al-A'raf 7:17)
Allaah the Most High informs us how Iblees (may Allaah curse him), has promised how he will strive to cause mankind to deviate from the Path of Allaah - Islaam. This is why no Muslim should remain complacent about being upon Eemaan, but rather should continue to strive in worshipping Allaah correctly till death. However, it is more obligatory that this worship be accompanied with the correct belief in Allaah and correct actions as legislated by Him, the Most High. It is therefore imperative to study those beliefs and actions that oppose Islaam and cause the one who adopts them to be eternally in the Hellfire (we ask Allaah for protection from it).
Nawaaqid ul-Islam is another great book by Shaykh ul-Islam, Muhammad bin Abdul-Wahhaab Rahimahullaah, which clarifies what the Tawheed of the Prophets and Messengers actually was. However, this work explains it from the angle of those actions which nullify one's Tawheed. Though there are many things that nullify Tawheed, the author highlights 10 main ones.
It is important to understand that these are foundations laid down to aid the believer in understanding how to preserve his ‘Aqeedah. For this reason, studying such a book under a scholar (such as the explainers of the book - Shaykh Fawzan, Shaykh Muhammad Amaan al-Jaamee, Shaykh Ubayd al-Jaabiree) is essential, as many political activists (those who adopt the kharijee, takfeeree ideology) use these principles to apply them to Muslims without understanding what a great sin it is to excommunicate another Muslim.
MANY SALAFI SCHOLARS EXPOUNDING UPON: Takfir and the Excuse of Ignorance - http://www.manhaj.com/manhaj/tags/takfir.cfm
Since this is an immensely important topic with principles that need to be clearly understood, different explanations have been included. Initially, I only found explanations of this valuable treatise in English by Hizbis, Takfiris or Majhool (unknown people). So more lectures will be added here when available Insha'Allaah.
You are requested to start your learning by going through the smaller explanations first.
Dr Abdulilah Lahmami Hafidhahullaah using the Sharh of Shaykh Ubayd al-Jaabiree Hafidhahullaah