It is anecdotally mentioned that when one says one is a seeker of knowledge, one is tested by asking them to:
- recite a juz' of the Qur'an,
- mention the full narration chain of a hadith in Sahih al-Bukhari or Sahih Muslim,
- talk about the differences between the opinions of the four schools of jurisprudence on a specific matter, or
- debate a topic of creed (e.g., one of the attributes of Allah) with a specific sect (e.g., Ash'aris)
If one says they need to refer to notes or materials, they need to reconsider their title as seekers of knowledge. Obviously, in the good old days, what was not memorized was not considered knowledge.
While this may seem daunting, on a more positive note, a good number of scholars achieved the above in a relatively short period of time as the value of knowledge in Islam cannot be overstressed. Those who know and those who do not know are by no means equal:
أَمَّنْ هُوَ قَانِتٌ آنَاءَ اللَّيْلِ سَاجِدًا وَقَائِمًا يَحْذَرُ الْآخِرَةَ وَيَرْجُو رَحْمَةَ رَبِّهِ ۗ قُلْ هَلْ يَسْتَوِي الَّذِينَ يَعْلَمُونَ وَالَّذِينَ لَا يَعْلَمُونَ ۗ إِنَّمَا يَتَذَكَّرُ أُولُو الْأَلْبَابِ
Is one who is devoutly obedient during periods of the night, prostrating and standing [in prayer], fearing the Hereafter and hoping for the mercy of his Lord, [like one who does not]? Say, "Are those who know equal to those who do not know?" Only they will remember [who are] people of understanding.
— Surah Az-Zumar [39:9]
Knowledge is not limited to only religious knowledge. In fact, the Prophet ﷺ encouraged worldly knowledge as much as he did with religious knowledge, let that be learning languages, geography, history, lineage, mathematics, etc. He mentioned that when a person dies, only three things continue to add to one's virtuous acts, and one of them is knowledge at large:
عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ: إِذَا مَاتَ الإِنْسَانُ انْقَطَعَ عَنْهُ عَمَلُهُ إِلاَّ مِنْ ثَلاَثَةٍ إِلاَّ مِنْ صَدَقَةٍ جَارِيَةٍ أَوْ عِلْمٍ يُنْتَفَعُ بِهِ أَوْ وَلَدٍ صَالِحٍ يَدْعُو لَهُ
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying: When a man dies, his acts come to an end, but three, recurring charity, or knowledge (by which people) benefit, or a pious son, who prays for him (for the deceased).
— Sahih Muslim, Book 25, Hadith 20
Accordingly, the Prophet ﷺ stressed that seeking knowledge is highly desirable to the extent that those who seek knowledge pave themselves an easy way to Jannah:
عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم: مَنْ سَلَكَ طَرِيقًا يَلْتَمِسُ فِيهِ عِلْمًا سَهَّلَ اللَّهُ لَهُ طَرِيقًا إِلَى الْجَنَّةِ
Narrated Abu Hurairah: that the Messenger of Allah (ﷺ) said: "Whoever takes a path upon which to obtain knowledge, Allah makes the path to Paradise easy for him."
— Jami' at-Tirmidhi, Book 41, Hadith 2
Since your question is specific to the knowledge of Islam, the Prophet ﷺ mentioned that when Allah wills the good of one, Allah makes one knowledgeable in religion:
عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، أَنَّهُ سَمِعَ مُعَاوِيَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم: مَنْ يُرِدِ اللَّهُ بِهِ خَيْرًا يُفَقِّهْهُ فِي الدِّينِ، وَاللَّهُ الْمُعْطِي وَأَنَا الْقَاسِمُ، وَلاَ تَزَالُ هَذِهِ الأُمَّةُ ظَاهِرِينَ عَلَى مَنْ خَالَفَهُمْ حَتَّى يَأْتِيَ أَمْرُ اللَّهِ وَهُمْ ظَاهِرُونَ
Narrated Muawiya: Allah's Messenger (ﷺ) said, "If Allah wants to do good for somebody, he makes him comprehend the Religion (i.e. Islam), and Allah is the Giver and I am Al-Qasim (i.e. the distributor), and this (Muslim) nation will remain victorious over their opponents, till Allah's Order comes and they will still be victorious."
— Sahih al-Bukhari, Book 57, Hadith 25
The Arabic word used in the hadith above is derived from the word faqaha (Arabic: فقه). This does not mean that religious knowledge is restricted to jurisprudence, but any knowledge that makes one able to address issues in Islam, let that be Qur'an, hadith, law, creed, jurisprudence, history, etc., is worth its while. There is no known disagreement among scholars, however, that the first book to learn is the Qur'an, then follow this by the Sunnah (starting with Sahih al-Bukhari and Sahih Muslim). The Prophet ﷺ said in Sahih al-Bukhari 96/9 that "the Qur'an was revealed and the people read the Qur'an, (and learned it from it) and also learned it from the Sunna." Ibn Taymiyyah said in Majmū' al-Fatāwa 24/54 that a seeker of knowledge should know what Allah commands and forbids, and to know this, one has to learn and memorize the Qur'an. In Tahthīb Iqtidā' as-Sirāt al-Mustaqīm 2/92), 'Umar ibn al-Khattāb said we should learn Arabic for it is part of the religion. In conclusion, the order of material to study is:
- Arabic language.
This leads to how scholars graded levels of knowledge. As-Suyūti classified seekers of knowledge into 911 levels of knowledge, which obviously is outside the scope of this forum, but it demonstrates that knowledge seeking is a topic that is highly researched. I will paraphrase the classifications of Ibn Kemal (see At-Takhrīj 'ind al-Fuqahā' wa al-Usūliyyīn pp. 301) that are based on the material listed above with the intention of knowing what Allah commands and forbids (as Ibn Taymiyyah said):
- Knowledge that enables one to have the knowledge to establish principles of jurisprudence and deduce rulings based on evidence without referring to any scholarly material other than their own. Examples of people in this level are Abu Hanifa, Malik, Ash-Shafi'i, Ahmad ibn Hanbal, Al-Layth ibn Sa'd, Al-Awza'i, etc.
- Knowledge that enables one to apply principles of jurisprudence of one of the schools of level 1 to deduce rulings based on evidence (Qur'an, Sunnah, ijmā', and qiyās) while referring to scholarly material of a specific madhhab, and agreeing or disagreeing with the owner of the school (level 1).
- Knowledge that enables one to apply principles of jurisprudence of one of the schools of level 1 to deduce rulings for matters that the owner of the school did not discuss.
- Followers of a scholar of level 1 that are not able to deduce rulings, but are able to classify and document knowledge (takhrīj) and are able to explain in details the reasoning behind the rulings deduced by the owners of the school in level 1.
- Followers of a scholar of level 1 that are neither able to deduce rulings nor able to classify and document knowledge, but are able to weigh alternatives and give preference or priority of one opinion over another based on the classifications done by scholars in level 4.
- Followers who have knowledge to differentiate between the weak and the authentic (e.g., in hadith or jurisprudence), understand generalization from specificity, absolute from restricted, etc.
- Followers who are able to understand material but are not able to discern strength from weakness, applicability, correctness, etc.
The term "seekers of knowledge" applies to all levels, but to put things in perspective, An-Nasafī (author of the book Kanz ad-Daqā'iq) is of the 6th category. Other scholars in the 6th category are 'Abdullah ibn Ibrāhīm al-Mahbūbi (author of the book Wiqāyt ar-Ruwāya fi Masā'il al-Hidāya, a famous book in the Hanafi school), Ibn Mawdūd al-Mūsili, and so on.
A journey of a thousand miles begins with a single step.