(Luke 3:1-22)
Luke saw the real beginning of the gospel (or good news) in the appearance of John the Baptist (cf. Acts 10:37). He gives us the historical context by giving its date and brief description of the political situation in Palestine. The ministry of John the Baptist marked the end of the old era (cf. Luke 16:16) as well as the beginning of the new. Luke saw John as the last of the prophets. His coming was in fulfillment of Isaiah 40:3-5.

Hebrew Law and Custom

The ministry of John the Baptist is considered to be the end of the old era as well as the beginning of the new. He was the last of the prophets (see Luke 16:16) and the fulfiller of prophecy. Isaiah 40:3-5 was fulfilled with John's coming.
(D. Guthrie and J. A. Motyer eds. The New Bible Commentary: Revised. Wm. B. Eerdmans Publishing Co., Grand Rapids, Michigan, 1970, 894.)

Historical Evidence

Because Luke is so precise about the government officials of the time, the beginning of John's ministry is datable. "There are, however, two alternative ways of reckoning the chronology of Tiberius' reign (A.D. 14-37) which make his fifteenth year either between A.D. 27 and 28 or AD 28 and 29. Pilate was governor of Judea A.D. 26-36; anda recently discovered inscription shows that his proper title was prefect and not procurator. Abilene lay to the north-east, within the general bounds of Palestine; Lysanias is mentioned in inscriptions."

Also, John's ministry can be dated because Annas and Caiaphas are mentioned as high priests. Even though there was only one high priest at a time, Annas, who held this office A.D. 6-15, was still very influential during the reign of Caiaphas, Annas' son-in-law, A.D. 18-37.
(The New Bible Commentary: Revised, 894)