Salih the son of Imam Ahmad narrates that Mutasim (the oppressive ruler of the time) turned towards my father (Imam Ahmad Rahimahullah) and commanded him to speak to Ibn Abi Duad (the misguided leader who was at the forefront of forcing deviation on the Muslim Ummah at the time). Imam Ahmad turned his face away and exclaimed, "How can I speak to one who I haven't ever seen at the door of an Alim!!"
(Imam Ahmad felt that because Ibn Abi Duad hadn't learnt Ilm from Ulama and was a self-taught scholar it was pointless to even try to make him understand the correct viewpoint and debate with him.)
(أدب الاختلاف، الشيخ عوامة حفظه الله ١٧٣)
Imam Ghazali mentions in Ihya:
A student of knowledge should not leave a subject from the praise worthy subjects (the essential sciences of Islamic knowledge) except that he has looked into it and has understood it's purpose and objective. (He has studied the subject with qualified teachers). Then if he is able to he should try to become an expert in it and if he isn't able to then (it isn't a problem and rather) he should seek the important Islamic sciences and understand them completely. He should learn from every Islamic science (the fundamentals of it). Indeed the Islamic sciences help each other out and are connected to each other.
The great Imam emphasizes here the importance of a student learning the fundamental of every essential Islamic sciences (Nahw Balaghah Aqeedah Fiqh etc.) before focusing on specializing in a specific science. This helps him become a proper well grounded Alim of the Deen.
معالم إرشادية ١٩٤
Mufti Muhammad Saad