The fifth phase: As Allah prepared for His Precious Book (The Qur'an) every century that large number of trustworthy memorizers to convey it in full from the Pious Predecessors to the Successors, Allah also prepared for the Sunnah the similar number or even more. Those trustworthy memorizers limited their long lifetimes to search for the authentic Sunnah of the Messenger of Allah (peace be upon him) that was reported from trustworthy narrators and up to the Prophet (peace be upon him). Those Trustworthy memorizers distincted the authentic from the weak or the fabricated and conveyed the Sunnah to us free of defects and doubts, then the matter was settled as such and everything became clear.
Because Allah has preserved the Sunnah of the Prophet (peace be upon him) as He preserved the Glorious Qur'an and made the Sunnah the fort, the shield, and the explanation of the Qur'an, the Sunnah became a thorn in the throats of atheists, the particle in the eyes of apostates, and the cutting sword to the doubts of the hypocrites.
So, it is not strange that they exert their utmost efforts to attack its authenticity, belittle its importance, and ward off people from sticking to and acting according to it so that they could do whatever they want with the Qur'an in order to destroy the religion. "They (the disbelievers, the Jews and the Christians) want to extinguish Allâh's Light (with which Muhammad peace be upon him has been sent? Islâmic Monotheism) with their mouths, but Allâh will not allow except that His Light should be perfected even though the Kâfirûn (disbelievers) hate (it)." (1)
 [Surat At-Tawbah: 72]. To see the refutation of this doubt, you can refer to the book of Tarikh As-Sunnah wa Mustalah Al Hadith p. 40-46, Researches on the dignified Sunnah p. 59-60, Shifa', images from the history of the Sunnah and the methods of the scholars of Hadith, p. 80-82.