Four statements of three Imaams of the Sunnah about the harm of the innovators..
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Imaam Ahmad (rahimahullaah) said:
There is none more harmful on the people of Islaam than the Jahmiyyah; they do not desire except to nullify the Qur’aan and the ahaadeeth of the Messenger (sallal-laahu-alayhi-wasallam) [Source: Tabaqaat Al-Hanaabilah 1/47]
Al-Haafifh Abdul Ghaniyy Al-Maqdisee (rahimahullaah) said:
Know—may Allaah have mercy upon you—that Islaam and its people are undermined by three groups; and the group that rejects the (authentic) ahaadeeth about the Attributes (of Allaah) and belie its narrators are more harmful to Islaam and its people than the disbelievers. [Source: Al-Iqtisaad Fil I’tiqaad page: 223]
Imaam Ibnul Jawzi (rahimahullaah) said: Abul Wafaa Ali Ibn Aqeel Al-Faqee (rahimahullaah) said:
The innovators (in Islaam) and the fabricators of ahaadeeth are more (harmful) than the Mul-hideen;  because the Mulhideen intend to corrupt the Religion (of Islaam) from outside, (but) these ones intend to corrupt the Religion (of Islaam) from within. They are like the people of a country who strive to corrupt its state of affairs, and the Mulhidoon are like besiegers from outside. So the insiders open up the fortress (for the besiegers); their evil is worse on the people of Islaam than those who do not ascribe themselves to it. [Source: Al-Mawdoo’aat 1/50]
Ibn Aqeel (rahimahullaah) also said:
There is none more harmful on the sharia than the innovators of the people of kalaam (i.e. the people of philosophical rhetoric) and the ignorant soofees. [Source: As-Sawaa-iq Al-Mursalah Alal Jahmiyyah Wal-Mu-attilah 4/1346]
 Mul-hideen: [The Mul-hideen are the people of Il-haad. Ilhaad means to turn away from that which one is obligated to believe in or do. This is a general definition of Il-haad and anyone who turns away from that which is obligated upon him to believe or do, then such a person is referred to as a Mul-hid. But Il-haad is of two types: Major Ilhaad and Minor Il-haad.
Complete Il-haad is to turn away from Islaam completely and that is Major Il-haad, which exits a person from the fold of Islaam, such as the Il-haad of the communists, the idol worshippers and those similar to them.
And Minor Il-haad does not exit a person from the Religion of Islaam, such as turning away from some actions. Allaah (The Most High) said: [وَمَن يُرِدْ فِيهِ بِإِلْحَادٍ بِظُلْمٍ نُذِقْهُ مِنْ عَذَابٍ أَلِيمٍ] ”And whoever inclines to evil actions therein or to do wrong, him We shall cause to taste a painful torment.” [22:25][for further details, see links http://www.ajurry.com/vb/showthread.php?t=13326 http://www.ibnothaimeen.com/all/noor/article_1802.shtml
Also see Tafseer of Soorah Al-Hajj Ayah 25 in ‘Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer kalaam Al-Mannaan’ of Imaam Sadi (rahimahullaah)