Since the middle of the 19th century, church periodicals in the form of Diocesan records occupied an important place in the social life of the Russian provinces, placing on their pages not only documents and addresses of church and official authorities, sermons and instructions, but also material of, for example journalistic, local history, historical and ethnographic nature. In light of this, the study of the process of Christianization of the Kazakh population of the Steppe Territory in the late 19th - early 20th century is of particular interest. The study considers materials of the “Omsk Diocesan Records”, the official periodical of the Omsk diocese, which until recently has been poorly studied in historiography. A comprehensive analysis of the source (the «Omsk Diocesan Records») showed that the Kazakh (Kyrgyz) population of the Steppe Territory and the Omsk diocese under the conditions of missionary activity of the Kyrgyz mission experienced a transformation not only in the ethno-religious, but also in the economic, social and cultural spheres. The “Omsk Diocesan Records” can be considered a full-fledged, original and versatile source about the history of the Christianization of the Kyrgyz (Kazakh) population of the Steppe Territory in the late 19th - early 20th century, reflecting the official position of the Russian Orthodox Church regarding missionary activity, its state, forms and methods, as well as certain aspects of the very process of Christianization of the Kirghiz (Kazakhs), such as: the socio-economic conditionality of the adoption of Orthodoxy, the process of socialization, acculturation and transformation of ethnic and religious consciousness of both the newly baptized and their recent relatives and newfound brothers in religion.