Luke 3:1  One again secular history crosses paths with faith. In this case, at least at this moment, there is no direct impact on Biblical events due to the Roman Empire, or the government of Pontius Pilate, so this is a marker. Tiberius Caesar had succeeded Caesar Augustus. The Pax Romana was stable.

Luke 3:2 Annas and Caiaphas were the high priests, but the word of The Lord came to John the son of Zacharias, who had previously been a priest, but John now lived in the wilderness.

Luke 3:3-6 The message that John brought to the people from The Lord was based on a quote from Isaiah 40:3-5. A voice crying in the wilderness - Luke says that John was that voice. Calling for people to repent, to make their paths straight. The metaphor of the rearrangement of the physical features of the wilderness is straightforward - making the crooked straight and so forth. And the promise - that all flesh will see God's glory. John recapitulated the ministry of the Old Testament prophets who called Israel to repentance. Their calls had often been rejected, resulting in defeat and captivity for Israel. Since Israel was no longer a sovereign nation, John's call was on a personal level. No doubt there was an eschatological expectation on the part of some of the listeners when John talked about all flesh seeing the glory of God. Expectations that God would intervene in history to grant Israel victory over her enemies and perhaps political sovereignty.

Luke 3:7-14  John's message is further elaborated. 1) Repent, because the true children of Abraham are not defined by physical descent; 2) Every tree that does not bear good fruit will be cut down and used for firewood. 3) The well-off should share with the needy; 4) Tax collectors should not cheat subjects of the Roman Empire; 5) Soldiers should not oppress subjects. This all sounds very legalistic, and I have to admit that the next few verses suggest that John believed that when the Messiah came (soon!) he would cleanse His threshing floor with fire, i.e. judge the (metaphorical) chaff.

Luke 3:15-18 John the Baptist stands as a bridge between the Old and New Covenants. He announces and in fact introduces the Messiah, but prior to this, he preaches a message of repentance and judgment. And he says that Messiah will cleans His threshing floor with the winnowing fork in His hand. John also foretells that the Messiah would baptize people with the Holy Spirit and fire. How much did John understand about the person and work of the Holy Spirit? Perhaps from personal experience, He understood that the Holy Spirit leads the believer, but this passage suggests that he did not understand grace or how the Messiah would die to save His people  from sin. Luke concludes his description of John's message is to describe it as the gospel, which seems odd. God's  judgment does not sound much like good news.