The origin of these abbreviations dates back to the 6th century AD, when Dionysius Exiguus, a Scythian monk, was commissioned by Pope John I to create a calendar that would accurately calculate the date of Easter. Dionysius based his calendar on the Julian calendar, which was the calendar in use at the time, and introduced the concept of numbering years from the birth of Jesus Christ. He used the Latin abbreviations AD and BC to denote years before and after the birth of Christ, respectively.
It is important to note that the terms AD and BC are not exclusive to the English language. They are used in many other languages around the world, including Latin, Italian, Spanish, French, and German. The reason why these abbreviations are still commonly used today is because they provide a convenient and universally understood way of denoting years in the Christian calendar.