The people in 200 A.D. did not know they were in 200 A.D. because the Anno Domini counting system for years was invented over 300 years later, in 525 A.D.

And it didn't become popular until hundreds of years later.

And, hundreds of years later still, the A.D. year-count was incorporated into the Gregorian calendar when it was invented in 1582 A.D.

Check out the 'Calendar' section of the FAQ: https://www.reddit.com/r/AskHistorians/wiki/language#wiki_calendars

The FAQ includes my previous answer explaining the origins of the Anno Domini calendar. Plus, there's this fine podcast by the handsomest historian that ever there was. :)