Why is it that one who does not believe in the movement of Ibn Abd al-Wahhab is not considered a Sunni, and are the Ash'aris not from Ahl al-Sunnah even though they and the Maturidis are the ones who founded the madhhab of Ahl al-Sunnah?

Adherence to the Sunnah (definition) is determined by conformity with the methodology of the Salaf. Schools of thought influenced by scholastic theology (Kalam) and philosophy, such as the Ash'aris and Maturidis, diverged from the methodology of the Salaf on important issues, despite being superior and closer to the Sunnah than other proponents of Kalam. Our stance on the Ash'aris is summarized as loving and commending them for what they agreed with the Salaf, and opposing and refuting their errors in what they differed from the Salaf. This methodology was followed by Imams of the Sunnah such as Ibn Taymiyyah, who dealt justly with the Ash'aris despite his criticisms of them, acknowledging their efforts in defending Islam, while they did not deal justly with him and hurled accusations at him. Sheikh Muhammad ibn Abd al-Wahhab followed in the footsteps of Ibn Taymiyyah, and he was subjected to harm and slander from the Ash'aris, and his call was a reformist Sunni call.