This dissertation is a multifaceted investigation of Christian martyrdom in late medieval Armenia (between the years of 1378 - 1425), in the Lake Van region, particularly in the administrative centre of Ostan, historically known as the district of Ṙštunik', in the Vaspurakan region. The questions explore the occurrences of martyrdom, the historical accuracy and context of the martyr accounts, and their theological significance and implications. The thesis introduces, for the first time in the English translation, the writings, compilations, and lives of two eminent doctors of the church in the Armenian tradition: Grigor Cerenc' Xlat'ec'i (1349 - 1425) and Matt'ēos J̌ułayec'i (1349 - 1411). In the year 1393 the Armenian church leader, Catholicos Zak'aria II (1369 - 1393) - the region's most prominent Armenian ecclesiastic - was martyred in Ostan (Persian, Vuṣtān; modern Gevaş, Turkey), on the southeast shore of Lake Van, under the rule of the Kurdish 'amīr' 'Izz al-Dīn Shīr and the Persian 'dānišman qāḍī'. The details of Zak'aria's martyrdom are now primarily extant in an anonymous martyr text, composed soon after the event. The anonymous author of Zak'aria's martyr text (a highly skilled 'vardapet') prefaces the text with a rich discourse on the influences of good and evil on the persons who shape the narrative. He appeals to Scripture and the Armenian tradition to reframe and repurpose the historical event of Zak'aria's death for his Christian audience who were prone to conversion. The Introduction to the thesis discusses the 'status quaestionis' on the martyr text of Catholicos Zak'aria and Armenian martyrology, and questions of authorship more generally before focusing on the liturgical role and function of martyr texts in the Armenian Menologium. Part one of the thesis, consisting of three chapters, lays out the historical context and administrative developments in the Lake Van region since the demise of the Mongol Ilkhanate in 1335. By examining the political, social, and ecclesial backgrounds to the martyr text of Catholicos Zak'aria, the investigation highlights contention between Kurdish, Turkman, Armenian, and subsequent Mongol and Timurid dynasties up to Ottoman annexation in the sixteenth century. Chapter Three offers the background to the Catholicosate of Ałt'amar, its ecclesiastical jurisdiction, and the function of the Ałt'amar island as a purely Christian polity. Furthermore, it introduces Catholicoi Zak'aria I and Zak'aria II and their leadership in relation to the Muslim authorities in Ostan, and the catholicosate's interface with the administration of Islamic law in the wider region. Chapter Four studies the socio-economic, religious, and juridical, life of the community in an Islamic society and in Islamic courts, treating the region's integration in commercial networks and the position of the Armenian merchant and artisan elite in the urban landscape. The thesis makes an original contribution to late medieval Armenian ecclesiastical, political, and social histories by studying the manuscripts where the martyrdom of Catholicos Zak'aria II is found, and by situating this event within the ecclesial, interreligious, and socio-economic contexts based upon primary sources. Part two examines the theological meaning of Zak'aria's martyr text. Christian - Muslim dialogue dominates the focus of Chapter Five, 'Apologetics and Martyrdom', which introduces Matt'ēos 'vardapet' J̌ułayec'i and his apologetic text and brings it together with the martyr text of Catholicos Zak'aria. The apology is a response to the challenges posed by the local Muslims, initiated by the encounter with the Persian 'dānišman qāḍī' on the island of Ałt'amar. Matt'ēos' apology and the martyr acts of this period were generated and grounded in the same context and mutually reinforced one another to express shared principles and common themes through different genres. The chapter showed that Matt'ēos and the authors of the martyr texts used the defining force of the genres - of apologia and of martyr acts - to affirm and confirm their religious practices and beliefs, specific to their world. For the Armenian ecclesial community, the martyrs offered the archetypal response to challenging times. In the face of an attempt to establish external rule over their lives, they presented and preserved their faith and its praxis. Chapter Six, 'Martyrdom as Illumination', explores the Armenian theology of martyrdom through a close reading of the theological discourse and the scriptural exegesis of Catholicos Zak'aria's martyr text, highlighting the themes and the language of the seven-branched lampstand, light, and virtues, as they appeal to Scripture and tradition. The Christian encounter with Muslims and Islam and the Armeno-Latin dialogues in this period shape the broader context and the development of the themes of good and evil, light and illumination, and virtue and vices. This thesis highlights the idea that when the texts of this period, in the milieu of the vardapets of the Lake Van region, are brought together, a threefold understanding of martyrdom emerges: martyrdom understood as illumination, apologetic, and resemblance of Christ's light in the person of the martyr. The investigation further sheds light on relations between Christians and Muslims on the Armenian Plateau in the late medieval period. Juxtaposing the histories and the encounters of the Muslim polities in relation to the Armenian ecclesial authorities and the demography and governance in the Lake Van region, the thesis contributes to our understanding of Armenian Christian and Turkish and Kurdish Muslim relations, and the ways that Christians in Greater Armenia dealt with changes at this time in history.