The Link between Religious Innovations and Disbelief

Imām Ibn Taimiyyah, may Allāh have mercy on him, said:

Al-Bidaʿ¹ are derived from al-kufr,² so there is not a [single] statement of a person of bidʿah, except that there is within it a branch from the branches of al-kufr.

¹plural of bidʿah: any act of worship not legislated by Allāh (tr.)
²denial and disbelief of the truth (tr.)

Minhāj Al-Sunnah Al-Nabawiyyah, vol. 6, p. 368.