How did Ibn Abd al-Wahhab derive the third nullification of Islam: "whoever does not make takfir to the mushrikin ... he has disbelieved"?

I'm interested in this quote from ibn Abd al-Wahhab (who is described as the founder of Wahhabism on Wikipedia): > ... the Third Nullification from the nullifications of Islām which the Shaykh Muhammad Ibn ‘Abdil-Wahhāb (Rahimahullāh Ta’āla) mentioned, as he said, “The Third: **Whoever does not make Takfīr to the mushrikīn , or doubts their kufr , or approves of their Math’hab, he has disbelieved.**“ > Nāsir Ibn Hamad Al-Fahd, Refraining From Making Takfīr of a Kāfir, in *The Evidences for the Ruling Regarding Alliance with the Infidels*, p.120. (pdf ) All ten nullifications are rebutted in Question 1 of *The Response* (by Al-Azhar scholars). However, they didn't directly describe the third nullification as inaccurate, but rather discuss the extremely high standards required for takfir. **Question**: How did ibn Abd al-Wahhab derive the third nullification of Islam: "whoever does not make takfir to the mushrikin ... he has disbelieved"? My impression is that it's hugely risky and pointless to engage in takfir: the only time it's correct is when it's so obvious they're basically engaging in self-takfir anyway (such as someone who says "I'm an ex-Muslim"). *Edit*: I have asked a question about what takfir actually is here: https://islam.stackexchange.com/q/47248/17163