The excellence of knowledge. The evidence for the excellence of knowledge in the Qur’an [is manifest] in the words of Allah:
“Allah bears witness that there is no Allah but He, and the angels, and men endued with knowledge, established in righteousness.” :16. See, then, how Allah has mentioned Himself first, the angels second, and men endowed with knowledge third. In this you really have honour, excellence, distinction and rank.
And again Allah said: “Allah will raise in rank those of you who believe as well as those who are given knowledge.” : 12
According to ibn-`Abbas, the learned men rank seven hundred grades above the believers; between each two of which is a distance five hundred years long. Said Allah. “Say, `shall those who know be deemed equal with those who do not?”  :12
Allah also said, “None fear Allah but the wise among His servants;” : 25
and again, “Say, `Allah is witness enough betwixt me and you, and whoever hath the knowledge of The Book!’ ” : 43.
This I mention to you in order to show that it was possible only through the power of knowledge. Allah also said, “But they to whom knowledge hath been given said, `Woe to you! The reward of Allah is better [for him who believes and does right],”  :80 showing thereby that the great importance of the hereafter is appreciated through knowledge.
And again Allah said, “These parables do we set forth for men: and none understands them save those who know.” : 42.
Allah also said, “But if they were to refer it to the Apostle and to those in authority amongst them, those of them who would elicit the information would know it” : 85
He thus made the knowledge of His will dependent upon their efforts to find it out, and placed them next to the prophets in the [ability] to make it known. It has been said that in the following words of Allah, “O Sons of Adam! We have sent down to you raiments wherewith to cover your nakedness, and splendid garments; but the raiment of piety-this is best,”  :25 the raiments represent knowledge, the splendid garments, truth, and the raiment of piety, modesty.
Allah also said, “And We have brought them a book: with knowledge have We explained it;” : 50 and again, “But it is clear sign in the hearts of those whom the knowledge hath reached;” :48 and, “With knowledge will We tell them;” : 6
and again, “[He] hath created man, [and] hath taught him articulate speech.” (55:2-3) This, however, He said reproachfully.
From: Al Ghazali’s – Ihya Ulumuddin (The Revival of Religious Sciences) – translated by Nabih Amin Faris