I get this question all the time in class. Here's the answer.
Events in the Bible are typically dated in reference to some epoch event or person. In biblical times events were dated according to “regnal years.” For example, if I were living at the time of David and my wife gave birth to a son, I would say that my son was born “in the 16th year of King David.” That would date his birth relative to a local event that we all know. We see such references, for example, throughout 1 & 2 Kings.
In New Testament times, the Roman Empire operated on a much larger scale than the reign of a tribal warlord or local king, dating events “from the foundation of the city of Rome,” or ad urbe condita (abbreviated A.U.C.), which correlates to 753 B.C.
Under the direction of Pope St. John I (A.D. 523-526), Dionysius Exiguus introduced the use of B.C. and A.D. With the Edict of Milan legitimizing Christianity in A.D. 313 and Theodosius I making it the official religion of the Roman Empire in A.D. 380, the birth of Christ became the “epoch event” used for dating. B.C. stands for before Christ, and A.D. stands for anno domini, or “in the year of our Lord” (not “after death”).
Although invented in A.D. 525, it took several centuries before B.C. and A.D. came into common usage. Today new terms have been introduced: Common Era (C.E.) and Before the Common Era (B.C.E.). In the 21st century we recognize that Christians are a minority of the world’s population, and we recognize that dating anno domini, “in the year of our Lord,” is not an appropriate reference for much of the world’s population, since they do not recognize Jesus as “Our Lord.” Although some criticize the new terms as being “politically correct,” C.E. and B.C.E. have become standard usage in scholarship and journalism for addressing a global audience that is 33% Christian, 21% Moslem, 16% nonreligious, 16% Hindu, 6% Buddhist, 0.22% Jewish and 8% Other.
So . . . if you’re writing for a Christian audience, it is certainly appropriate to use B.C. and A.D. (I do so on my web site, blog and for explicitly Christian publications); if you’re writing for a more general audience (say, for a newspaper or an academic or scientific journal) use B.C.E. and C.E.
Dionysius Exiguus, by the way, was a very interesting character. His name means “Dionysius (or Dennis) the Small (or “lesser”),” a self-proclaimed nomenclature that distinguishes him from Dionysius, a member of the Areopagus, mentioned in Acts 17: 34, who became one of the few believers in Athens. Tradition views this Dionysius (the Areopagite) as a martyr and the first bishop of Athens.
Our Dionysius, however, was one of the great scholars of the 6th century, extraordinarily learned in Scripture, well versed in canon law and a tireless translator who rendered many Greek texts into Latin, preserving significant works whose Greek originals have been lost. Dionysius Exiguus (or as one evangelical wag called him, “Dennis the Dwarf!”) wrote a collection of 401 ecclesiastical canons, including apostolic canons and decrees of the councils of Nicaea, Constantinople, Chalcedon and Sardis. His friend, Cassiodorus (c. 485-585), states that Dionysius was from Scytha, a region near the Black Sea, and he refers to him simply as “a monk,” although the Venerable Bede in his Ecclesiastical History refers to him as an “abbot.”
Dionysius’s works appear in Migne’s Patrilogia Latina, vol. 67. He died sometime before A.D. 550.