Interdisciplinary studies are among the new topics in science. They are the outcome of the complexity of problems and the nonlinearity of social relations between different fields of knowledge, which has made it almost impossible to understand them through interdisciplinary methods. The humanities also need to use interdisciplinary methods to advance their goals because the nonlinearity of relationships is more pronounced in them. One of these methods is the use of cognitive studies, especially cognitive linguistics. In this field, a new theory of "conceptual metaphor" can be used in religious studies. In the present study, first, authors introduced the theory and explained the necessity of its application in Nahj al-Balaghah studies; then, by examining the concept of "speech" in Nahj al-Balaghah based on the theory of conceptual metaphor studied the effect of this view on the way of understanding religious propositions. The main question is how the concept of "speech" is conceptualized in Nahj al-Balaghah, and its understanding can affect which areas of knowledge. For this purpose, first, all expressions containing the word "promise" were extracted, and the origins of its conceptual metaphor were examined. Examination of metaphorical expressions related to this word indicated that firstly, "speech" in Nahj al-Balaghah was structured using several concepts of origin such as object and place. Furthermore, examining metaphorical expressions containing the meaning of speech, it was indicated that this linguistic feature had been used to explain abstract concepts in Nahj al-Balaghah, such as the effects and consequences of speech, obedience, the importance of timely speech, lies, controversy, adaptation of words and deeds. Secondly, the semantic expansion of the concept of "speech" in Nahj al-Balaghah has taken place in the light of contemporary metaphorical theory.