He was born on January 7, 1873, in the village of Protasev Ugol, Sapozhovsky County, in the Ryazan Governorate of the Russian Empire, in a family of believer merchants. His grandfather was a serf peasant who had managed to buy himself out, had begun to trade, and had accumulated considerable wealth. Over the years, however, trade did not go well, and the family was forced to pledge and subsequently lose much of their assets. In 1881, Posnov began to study in a rural school, where, due to the financial difficulties of his family, he could receive only a basic education, but the village priest advised his father to send him to a theological school. The family followed the advice and in 1883 Mikhail enrolled at the preparatory class of the Sapozhok Ecclesiastical School. In 1888, he moved to the Ryazan Theological Seminary and in 1894 passed the entrance examination for the Kiev Theological Academy.
In 1903, Posnov graduated from the Kiev Theological Academy, where his tutors were Vasiliy Bolotov and Alexander Briliantov. Soon after graduation he started lecturing in church history and held office of an assistant inspector at the Academy. He was appointed associate professor in 1908, and since 1910 has taught the New Testament Scriptures in the Second Department. He became professor of church history in 1913. For his services to the Church, he was awarded Order of St Anna, 2nd and 3rd degrees, and Order of St Stanislav, 2nd and 3rd degrees.
In 1917, the Civil War and the national movement in Ukraine forced him to move to Odessa, where he obtained a job as a lecturer at the Novorossiysk University. Despite the initial welcome he enjoyed, Posnov felt compelled to leave the country. He joined a large group of emigrants who arrived in Thessalonica and from there hewent to Serbia. At the end of 1919, Posnov decided to immigrate to Bulgaria and arrived in Sofia. He was accepted to teach dogmatics and church history at the Sofia Theological Seminary, and held lectures in Plovdiv as well. In the same year he was invited to be a professor of church history at the Theological Faculty of Sofia University of St Kliment Ohridski, where he taught until 1928. He was also a part-time lecturer at the Kyustendil and Second Sofia Male High Schools. Posnov died in Sofia on October 13, 1931.
The result of Posnov’s studies on the foundation for the life of the Jewish people after the Babylonian captivity was a series of publications in the Proceedings of the Kiev Theological Academy, as well as the extensive monograph Иудейство. К характеристике внутренней жизни еврейского народа в послепленное время [Judaism. Towards a Characterization of the Inner Life of the Jewish People in the Post-Captivity Period] (1906), which discusses issues of Jewish messianism, the appearance of currents and sects in Judaism, the problems of men in the Great Synagogue (the assembly of elders under the leadership of Ezra), and the emergence of apocalyptic literature . In his doctoral thesis, Гностицизм II века и победа Христианской Церкви над ним [Gnosticism of the Second Century and the Victory of the Christian Church over It] (1917), Posnov examined the nature of Gnosticism and its influence on the early Christian Church and formed his stand that the development of Gnostic ideas had to do with Samaritan religious views, and, Gnosticism could be seen as a natural reaction of the pagan world to Christianity. At the same time, Gnosticism was, in essence, a religious worldview that sought to displace Christianity and take its place. He emphasized on the importance of Gnosticism in accelerating the development and the final formation of the organization of the Church at the end of the 2nd century AD.
Posnov also drew attention to current problems of his time in his works: К вопросу об учреждении богословских факультетов [On the Issue of Establishing Theological Faculties] (1906), О римском католицизме [On Roman Catholicism] (1908), Человеческий труд с библейской и культурной точки зрения [Human Work from Biblical and Cultural Perspective] (1908), and others. In the work Первая христианская община и коммунизм [The First Christian Community and Communism] (1909) and О личности основателя христианской церкви: изложение и краткий разбор рационалистических, мифологических и натуралистических воззрений на лицо И. Христа [On the Person of the Founder of the Christian Church: An Exposition and Brief Analysis of the Rationalist, Mythological, and Naturalistic Perspectives of Jesus Christ] (1910). Posnov examined critically the rationalist and naturalistic views of H. E. G. Paulus, D. F. Strauss, J. E. Renan and other representatives of negative biblical criticism. He wrote a large number of articles on biblical history for the Orthodox Theological Encyclopaedia. In Bulgaria he wrote and published his fundamental work История Христианской Церкви (до разделения церквей – 1054 г.) [History of the Christian Church (until the split of the churches –1054)], which was posthumously published for the first time in Bulgarian, with the title: История на Християнската църква. Т. I-II. До разделянето на църквите (1054 г.) [History of the Christian Church. Vol. I–II. Until the Split of the Churches (1054)] (1933), subsequently published in Russian in Belgium (1964) and republished in Moscow (2005).
His scientific research was focused mainly on the history of the Church and was divided into works before his emigration to Bulgaria and afterwards. The following studies were published in the Russian Empire:
- Идея Завета Бога с израильским народом в Ветхом завете: (опыт богословско-философского обозрения истории израильского народа). (Магистърска теза) Богуслав, 1902.
- Взаимодействие двух факторов в истории израильского народа – божественного и человеческого. Киев, 1903.
- Слово в день св. Иоанна Богослова. Любовь, как начало истинного Боговедения и основа христианской просветительской деятельности. – Труды Киевской духовной академии, Киев, 1903, № 10, 230–237.
- К вопросу об учреждении богословских факультетов. – Труды Киевской Духовной Академии, Киев, 1906, № 4, 667–688.
- Иудейство. К характеристике внутренней жизни еврейского народа в послепленное время. Киев, 1906.
- К вопросу об источниках христианского вероучения и задачах его. – Християнское чтение, Санкт Петербург, 1906, № 11, 773–800.
- О судьбах библейского Израиля. Киев, 1907.
- О римском католицизме. Киев, 1908.
- Новые типы построения древней истории Церкви. – Християнское чтение, Санкт Петербург, 1908, № 11, 1487–1501.
- Человеческий труд с библейской и культурной точки зрения. Киев, 1908.
- Первая христианская община и коммунизм. – Християнское чтение, Санкт Петербург, 1909, № 4, 588–602.
- О личности основателя христианской церкви: изложение и краткий разбор рационалистических, мифологических и натуралистических воззрений на лицо И. Христа. – Християнское чтение, Санкт Петербург, 1910, № 5–6, 732–760.
- Евангелие Иисуса Христа и Евангелие апостолов о Христе. Киев, 1911.
- Самарийские маги – христианские ересиархи. Петроград, 1915.
- Гностицизм II века и победа христианской церкви над ним. (Докторска диссертация) Киев, 1917.
- Митрополит Антоний как православный богослов-догматист. Варшава, 1929.
After his arrival in Bulgaria, Posnov scrutinized his research on the problems of the Ecumenical Councils and in the field of Christian dogmatics and developed other topics as well:
- История на християнската църква. – Годишник на Софийския университет, I. Богословски факултет. София, 1923/25, 143–162.
- Първият Вселенски Никейски събор. – Годишник на Софийския университет, II. Богословски факултет. София, 1924/25, 1–90.
- Първият Вселенски събор в Никея и значението му. (По случай 1600 г. – 325 – 1925). – Църковен вестник, XXVI, 1925, №23, 1.
- Проблеми и въпроси, повдигнати в западната богословска литература по повод 1600 г. юбилей на Никейския събор (325 г.). – Годишник на Софийския университет, III. Богословски факултет. София, 1925/26, 139–150.
- Няколко думи за прима̀та на Римския папа. – Годишник на Софийския университет, III. Богословски факултет. София, 1925/26, 151–163.
- Монашеството през IV-XI векове. – Годишник на Софийския университет, IV. Богословски факултет. София, 1926/27, 63–104.
- Сардикийският събор и неговата католическа дейност. – Годишник на Софийския университет. IV. Богословски факултет. София, 1926/27, 105–128.
- Византинизъм в историята на древната християнска църква. – Годишник на Софийския университет. V. Богословски факултет. София, 1927/28, 217–244.
- Историческа справка по въпроса за състава на древните събори. – Годишник на Софийския университет. V. Богословски факултет. София, 1927/28, 245–258.
- Император Александър II като освободител на руския и българския народ. – Църковен вестник, XXIX, 1928, № 9, 102–103.
- Славянските народи и християнската църква. (Общ очерк). – Духовна култура, 1928, № 36, 19–29.
- История на Християнската църква. Т. I-II. До разделянето на църквите (1054 г.). София, 1933.