The People of bidah do not rely on the Book, the Sunnah and the narrations of Sahaabah and the Taabi-een; rather they rely on the intellect and the (Arabic) language. You find them not relying on the transmitted books of tafseer, the ahaadeeth (of the Prophet) and the narrations of the salaf, rather they rely on the books of science and philosophy compiled by their figure heads. This is also the way of the atheists: indeed they hold onto the books of philosophy, science and language. As for the books (compiled) from the Qur’aan, the narrations (of the Prophet) and the narrations (of the salaf), they do not pay attention to them. These ones (i.e. the atheists) turn away from the texts of the Prophets, because for them (such texts) do not constitute knowledge. And those ones (i.e. the people of bidah) explain the Qur’aan with their views and understanding without narrations from the Prophet and his companions. Imaam Ahmad (rahimahullaah) said: Most of the erring of the people (occurs) from the angle of false interpretation and analogy.
[Source: Tareequl Wusool Ilal Ilmil Mamool Bi-Marifatil Qawaa-id Wad-dawaabit Wal-Usool: of Imaam Sadi (rahimahullaah)|Translated by:Abu Mu-aawiyyah (Abdullaah Al-Gambi)]