Eusebius of Caesarea (as opposed to Eusebius the bishop of Nicomedia) was born circa
A.D. 260 and is best known as the Father of Church History. He wrote a history of Christianity covering the first three centuries among many other important works. His work was enabled by his position as a research librarian in a large private library of some 30,000 volumes. His patron, Pamphilus, was tortured, imprisoned, and martyred in 303 before the rise of Christianity under Constantine whereupon Eusebius was made bishop of Caesarea. Upon the death of Constantine, Eusebius began writing his autobiography which was interrupted by his own death approximately two years later at the age of almost 80. We are indebted to the writings of Eusebius for much of what we know about the early Christian church.1
1 Glen F. Chestnut, Eusebius of Caesarea, in David Noel Freeman, ed., The Anchor Bible Dictionary (New York, NY: Doubleday, 1996, c1992), 2:673-676.