It is the knowledge regarding Allah, His names and attributes, knowledge of His rights over His creation, and what He the Most High has prescribed for them.
It is the detailed knowledge of the path that leads to Allah, knowledge of the purpose of our creation, and the end in which the slave will result.
One who possesses this knowledge knows what Allah has made lawful for him and what He has prohibited him from, what pleases Him and what evokes His anger.
Amongst those who have taken upon themselves to observe the precepts of the religion will result in Paradise and bliss, and the rest will result in a place of disgrace, humiliation and misery.
Seeking knowledge of the religion helps to achieve Taqwa (fear) from Allah and increases Iman. Allah said:
“It is only those who have knowledge amongst His slaves that fear Allah.” [Soorah Al-Fatir (35): 28]
And Allah reminds us the main purpose of the creation of mankind saying: "And I created not the jinn and men except that they should worship Me." [Soorah Az-Zariyat (51): 56]
Prophet Muhammad (sallallahu alaihi wa-sallam) said: "Whoever treads on a path in search of Islamic knowledge, Allah will ease the way to Paradise for him. The angels will lower their wings, pleased with this seeker of knowledge, and everyone in the heavens and on earth will ask forgiveness for the knowledgeable person, even the fish in the deepest of waters will ask for his forgiveness” [Abu Dawud, Ibn Majah, Tirmidhi # 2835-sahih hadith]
Allah has honored and extolled those who posses knowledge, and raised their status high, He made them bear witness to His Oneness with sincerity and devotion.
“Allah will exalt in degree those of you who believe and those who have been granted knowledge.” [Soorah Mujadilah (58): 11]
Knowledge is a sign of goodness, happiness and success. As the Prophet Muhammad (sallallahu alaihi wa-sallam) is reported to have said:"If Allah wants to favor someone, He grants him comprehension (understanding) of this religion." [Sahih Bukhari vol.1 # 71, Tirmidhi and Musnad Ahmad]