1st century

The  was of great importance to the Doctor and his companions. The First Doctor, Ian and Barbara made at least three separate trips to the century. (DW: The Romans, PDA: Byzantium!, ST: The Last Days) Meanwhile, the Seventh and Tenth Doctors were both known to be in Pompeii for the eruption of Mount Vesuvius, as was Jack Harkness, apparently more than once. (BFA: The Fires of Vulcan, DW: The Fires of Pompeii, DW: The Doctor Dances). Long after Harkness' trips to Pompeii, he would become entombed for about two millennia, starting in this century. (TW: Exit Wounds)

Rome vs. Jesus Christ
The first century was dominated by the birth and life of Jesus Christ, the founder of the eponymous religion. The birth took place in a stable next to an inn in Nazareth, (PDA: Byzantium!) because the Doctor had taken the last room in the inn. (DW: Voyage of the Damned)

This event was so monumental that many nations, particularly those of Europe and the Middle East, reoriented their calendars to consider the time before his birth as BC, and the time after his birth as AD, or Anno Domini. (DW: Earthshock) Towards the middle of the century, Christ was crucified, and, according to some, rose again from the dead. This miracle spawned many tales of his godhood, and was ultimately the fact at the core of Christianity. Increasingly, people — mostly former Jews in Judea and Syria, the places where Jesus had principally lived —began to believe in his divinity. By the latter half of the century, the religion had spread throughout the lands of the Mediterranean, thanks to evangelism by his former disciples like Paul of Tarsus, Barnabas the priest, Mark of Jerusalem, Peter of Galilee, Matthew the tax official and Luke the physician. (PDA: Byzantium!)

A countervaling force was that of the Roman Empire. In part because it was polytheistic and featured state-sponsored occultism, (DW: The Fires of Pompeii) the government of Rome tended to view Christians and Jews with suspicion. Under the leadership of Caesars Augustus and Tiberius, Christianity was squelched, often by using Jewish leaders against their members who had converted to Christianity. (PDA: Byzantium!) Even by the time of the later Caesar, Nero, at least some Christians who were Roman citizens chose to keep their religion hidden. (DW: The Romans)

The lure of a good volcano
The eruption of Mount Vesuvius in the middle of the century was an event labelled by the Tenth Doctor as a fixed point in time — one of the immutable events that had to happen in order for the Web of Time to maintain its integrity. Possibly for this reason, it was a sort of magnet for the TARDIS, which brought two different incarnations to witness Vesuvius' destruction of the Roman city of Pompeii. Both the Seventh Doctor and Mel and the Tenth Doctor and Donna were present in Pompeii at the time of the mountain's explosion — but their paths never crossed. The Seventh Doctor's TARDIS was buried by the volcano — something that would later cause confusion for the Fifth Doctor when it was excavated in the 20th century by UNIT. Meanwhile the Tenth Doctor discovered that he was directly responsible for the volcano's eruption. (DW: The Fires of Pompeii; BFA: The Fires of Vulcan)

Immortality tested
During this century, the Ninth Doctor's former companion, Jack Harkness, was buried underneath what would become Cardiff. He stayed underground, dying and returning to life, until he was eventually exhumed in the early 20th century. (TW: Exit Wounds)