Religion is an important aspect of the life of man and the society. This is because it is an important tool for relations within the society. Religion brings unity among people and the society at large. The unity of the church is of utmost importance for Christian religion and is, therefore, non-negotiable given that it determines the church. However, cultural and language differences exist even within churches of the same belief. So, in the church, should diversity violate unity and vice versa? The function of language in the church in the light of unity and diversity is therefore important. Which language should be used for the sake of the unity of the church in a diversified multilingual Kenyan environment? English, Kiswahili, local languages or an amalgamation of the three? Given that preaching is a social act, the study to which this paper is based adopts a descriptive design and discusses the social functions of language in religious sermons, showing how diversity in language is used in church to foster or hamper unity. The study demonstrates that different languages are used to elicit different social responses, thereby achieving different social functions and effects in the coordination of church activities. The ultimate goal being that respect for each other leads to recognition of the other, culminating into deep Christian confidence. Language is, therefore, used to bring a diversified people together as a unified community of faith, hence promoting unity in diversity in the church.