[1.9 The Need for Beneficial Knowledge]
This is because the person, even if he has believed that Muhammad is the Messenger of Allah and that the Qur’an is the truth in a general way, is commonly in need of knowledge of that which would benefit him and harm him. He is in need of knowledge concerning what he has been commanded to do and forbidden from doing in the finer aspects of the matters and in those areas of which he has no knowledge. [Not only this but we find that] that which he does have knowledge of, he does not put the greater part of it to practice! Assuming that all of the commands and prohibitions contained in the Qur’an and Sunnah have reached him, then the Qur’an and Sunnah contain laws that are general and universal for which it is not possible to specify to every individual person - therefore the person has been commanded due to the likes of this to ask for guidance to the Straight Path.
Guidance to the Straight Path includes all of the following matters: cognizance of what the Messenger (pbuh) came with in detail, cognizance of what comes under his general orders and concern for acting according to ones knowledge, for indeed just having knowledge is not a cause for attaining guidance if one does not act according to his knowledge. This is why He said to His Prophet after the treaty of Hudaybiyyah,
"Indeed We have given you a manifest victory. That Allah may forgive you your sins of the past and future, complete His Favour upon you, and guide you on a Straight Path."
[Surah Fath (48): 1-2]
"And He said with respect to Musa and Harun,
and We gave them the clear Scripture, and guided them to the Right Path."
[Surah as-Saffat (37): 117-118]
The Muslims have differed as to what Allah Willed from the textual matters - matters of knowledge, belief and action while all of them are agreed that Muhammad is the truth and the Quran is the truth. If all of them were to have attained guidance to the Straight Path in totality then they would never have differed. Furthermore the majority of those who know what Allah has ordered disobey Him and do not follow His Way. If they were guided to the Straight Path in these matters then they certainly would have performed what they had been commanded to do, and left what they had been forbidden from. As for those whom Allah guided from amongst this nation until they became from the God-Fearing Friends of Allah, then the greatest reason for this was their supplicating to Allah with this supplication,
"Guide us to the Straight Path." [Surah al-Fatihah (1): 6]
in every prayer along with the knowledge of their continuous need of Allah that He guide them on the Straight Path, So due to their continually saying this supplication and their acknowledging their continuous need of Him they became God-Fearing Friends of Allah.
Sahl bin ‘Abdullah at-Tustori said,
"There is not route between a servant and Allah closer to Him then need."
The one who has attained guidance in the past is in need of guidance in the future, this is the real meaning behind the saying of those who say that it means: ‘establish us and guide us to being firm upon the Straight Path.’ The opinion of those who say that it means: ‘increase us in guidance’ includes what has preceded. But all that has been stated refers to His guidance to the Straight Path that is to be granted in the future, for indeed action in the future is upon knowledge that is not yet attained. And the person is not considered to be one who is guided until he acts according to his knowledge in the future, but it is possible that this knowledge not be there in the future, rather it could be removed from the heart, and if it still be there it is also possible that it not be acted upon. Therefore all of mankind is in dire need of this supplication, this is why Allah made it obligatory upon them in every prayer and they are not in need of any other supplication as they are of this one. When guidance is obtained to the Straight Path then help, provision and all of the happiness that the soul seeks are obtained [from Allah]. Allah knows best.
Extract from DISEASES OF THE HEARTS & THEIR CURES by Shaykhul Islam Ibn Taymiyyah (translated by Abu Rumaysah)