Imaam Ibn Qayyim (rahimahullaah Ta’aala) said:
If bid’ah only consisted of lies, then it would NOT have been accepted and everybody would even hurry to condemn it and refute it. And if it only consisted of Haqq (truth), then it would not have been a Bid’ah, rather, in agreement with the Sunnah. But it (Bid’ah) consists of both truth and lies in which the truth is clothed with lies.
[as-Sawaa´iq al-Mursalah (3/925)]