Eusebius of Caesarea was a Christian historian and theologian who lived during the early 4th century. He is best known for his work, the "Ecclesiastical History," which chronicles the development of the Christian Church from its origins to his own time. Eusebius is often referred to as the "Father of Church History" for his detailed accounts of early Christian leaders and events. In addition to his historical writings, Eusebius played a significant role in the establishment of the Christian canon. He was involved in the First Council of Nicaea in 325 AD, which addressed key theological issues and helped shape early Christian doctrine. His contributions have had a lasting impact on both history and theology.