Ibn Al-Jawzee (rahimahullaah) said:

“Abul-Wafaa ‘Alee Ibn ‘Aqeel said: ‘Our Shaikh Abul-Fadl Al-Hamdhaanee said:

‘The innovators in Islaam and the fabricators of ahaadeeth are worse than the disbelievers. This is because the disbelievers attempt to corrupt the Religion from the outside, whereas these individuals attempt to corrupt it from the inside. They take the similitude of the inhabitants of a land, who strive to corrupt its condition (from the inside), while the disbelievers take the similitude of raiders laying siege to the land from the outside. Thus it is the ones on the inside that open the doors of the land’s surrounding barrier (and let the besiegers in). These types (of people) are far worse to Islaam than the ones who don’t attribute themselves to it.”

[Al-Mawdoo’aat: 1/51]
 
 
Shaikh-ul-Islaam Ibn Taimiyyah, may Allaah have mercy on him, said:

“If it were not for those whom Allaah placed to repel the danger of them [meaning the innovators], the Religion would have been corrupted. Their corruption is worse than the corruption caused by the conquering of the enemies at times of war. For indeed, when they conquer, they do not corrupt the hearts or the Religions found in them until perhaps a long time has passed. But as for these people [i.e. innovators], they corrupt the hearts from the very beginning.”

[Majmoo’-ul-Fataawaa (28/232)]