Ibn Rajab (Allah grant him mercy) wrote:
It is known that worship is built from only three sources: fear, hope, and love. Each one is an intrinsic duty, and combining the three is an obligatory injunction.
Because of this, the Forbears censured whoever devoted oneself to one while neglecting the other two. [After all,] the innovation of the Khawarij and those resembling them came about from emphasizing fear while avoiding love and hope; the innovation of the Murji’a [those who believe that man guarantees the unrepentant safety from punishment for grave sins] came about from clinging to hope alone while avoiding fear; and the innovation of many advocates of freethinking [ibahiya] and divine indwelling [hulul] who ascribe themselves to devotion came about from singling out love while avoiding fear and hope.
From Istinsh q Nas m al-Uns Min Nafah t Riy d al-Quds, pp25-26.