The name which gives knowledge and recognition of the Ahkaam and practical matters, with their evidences, as “Fiqh”, and they named that which gives the conditions of proofs, in brief, in providing Ahkaam (rulings) as Usoolul Fiqh (Principles of jurisprudence) and they named the knowledge of Aqaaid as, with its evidences, as “Al-Kalaam” (or ‘Ilm-ul-Kalaam), because the subject of its investigations or subject matters was their saying, “the speech in so and so (matters) and because the matter of Kalaam ( ‘Ilmul Kalaam-literally- The knowledge of speech) was the most famous of its investigations or subject-matters and the field in which there was the most disagreement and argumentation, so much so that some of the leaders who took power by force killed many of the Ahl-ul-Haqq (People of the Truth) because the Ahl-ul-Haqq refused to proclaim that the Qur’aan is created (a belief of the Mu’tazilah sect).
Another reason as to why this field (i.e., specialization in ‘Aqeedah) was termed ‘Ilm-ul-Kalaam is because it results in a person becoming very strong in speaking in determining matters of the Sharee’ah, in silencing opponents, like mantiq (logic) of the philosophers, and also because it was the first field of ‘Ilm which was learnt and taught through kalaam (speaking) alone, and thus this term of “Kalaam” is exclusive to it and was not used for other than it, for the purpose of distinction.
Another reason behind it being called ‘ilm-ul-Kalaam because it is established through mutual research and involves a discussion from two angles and with regards to other sciences, it can be acquired through pondering and researching.Another reason for it being called ‘Ilm-ul-Kalaam because from all the sciences, it is the one in which there is the most argumentation and differences of opinion, and thus it became severely in need of ‘kalaam” (speaking) with the opponents and refuting them.
Another reason for the science of Aqaaid being called ‘Ilm-ul-Kalaam is because, due to its proofs being so strong, it became as though it alone was the “Kalaam” (speech), in exclusion from what was other than it, like how it is said regarding the stronger one from two speeches: “This is the speech” (kalaam).”
Also it is called ‘Ilm-ul-Kalaam based upon qat’iyy evidences (i.e., evidences that are absolute, decisive, and firmly established in their nature) most of which are aided by evidences that are sam’iyyah (based on hearing), it became -from the ‘Uloom- the one that had the greatest effect in penetrating the hearts and permeating them, so it was named “al-Kalaam”, derived from “al-Kalm”, which means a wound.
Aqaaidun-Nasafiyyah by Allamah Taftaazaani