What does Mark 1:5 mean?

When the people hear John's call to repent for the forgiveness of sins, and to become ceremonially clean by baptism in the Jordan River, they respond in droves. Despite being in the "wilderness," Mark notes that "all the country of Judea and all Jerusalem" came to John. This figure of speech doesn't mean literally every single person from Judea and Jerusalem came to be baptized; it just means that people from every area came—all areas were represented. Hundreds and perhaps even several thousand people were baptized by John in the late AD 20s.

This is not the first time Jews had repented of their sins en masse. In Judges 10, God allows the Philistines and Ammonites to terrorize the Israelites because of the Israelites' idolatry. In Judges 10:10, the Israelites cry out, "We have sinned against you, because we have forsaken our God and have served the Baals." God instructs them to get rid of their idols. They do, and God allows Jephthah to rescue them (Judges 11:32–33). In 1 Samuel 7:1–14, Samuel orders the Israelites to get rid of their idols, and God rescues them from the Philistines again.

At the time of John the Baptist, the Jewish nation is occupied by the Romans and the people are looking for the Messiah to deliver them. As John's call for repentance winds down, Jesus the Messiah comes—not merely to deliver them from the Romans, but from their sin-induced separation from God.

Like the prophets of the Old Testament, John is not universally received. The sheer number of people who come to hear him teach attracts the attention of the Jewish leaders in Jerusalem (Matthew 3:7; John 1:19–28). This publicity eventually results in his death when he speaks out against Herod's incestuous marriage, and Herod's wife schemes to have John beheaded (Mark 6:14–29). Acknowledging that our own sins are wrong and working to turn away from them sounds like a reasonable thing to do, even to those who do not believe. But faithfully following God serves as a witness to reveal the sinfulness of others (Hebrews 11:7). That is not welcomed by a world that lives in rebellion to God.