Mark 1:4-5
4 John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.
5 And there went out unto him all the land of Judaea, and they of Jerusalem, and were all baptized of him in the river of Jordan, confessing their sins.
The Grace New Testament Commentary - Mark 1:4-5
1:4 John is identified as the messenger who appears in the wilderness. The wilderness is where Israel expected the messenger to appear. Although the temple was in Jerusalem, Ezekiel had documented the glory of God leaving the city (Ezek 11:23). Jerusalem was hostile toward Jesus throughout the Gospels (cf. Matt 23:37) and because of its corruptness, God is calling the people into the wilderness to identify with His prophet.
Baptism became such a trademark of John's ministry that he became known as John the Baptizer. His ministry involved a baptism of repentance for the remission of sins. Many equate this with the message of eternal life. In this view repentance is a synonym for faith resulting in justification, with baptism as the first step of discipleship.
However, this view is flawed. First, repentance and faith are not synonyms (cf. Mark 1:15; Acts 20:21). Second, such attempts ignore the historical offer of the kingdom to Israel. Third, the Apostle Paul's interpretation of John's baptism identifies it as temporary and preparatory (cf. Acts 19:1-7).
A better view is to understand that John was calling Israel to repent in order to restore fellowship with God and receive the kingdom. While repentance does not grant eternal life, it prepares their hearts to receive the Messiah (cf. Acts 19:4). The evidence of repentance was expressed by water baptism. The genitive metanoias could function as a genitive of source, that is, "a baptism arising out of repentance," or of description, "a baptism characterized by repentance."
1:5 The appearance of John resulted in a response from all of Judea and those from Jerusalem. Those John baptized were confessing their sins.
Thomas Constable's Expository Notes on the Bible - Mark 1:4
1:4 John baptized people when they gave evidence of "repentance" (Gr. metanoia). "A baptism of repentance" means a baptism characterized by repentance. The Jews whom John baptized not only changed their minds, the basic meaning of metanoia, but they also changed their behavior. This is the only occurrence of metanoia in Mark. The change in their thinking - regarding their sinfulness and the necessity to get right with God in view of the messianic kingdom being at hand - resulted in the forgiveness of their sins (cf. Matt. 12:41; Acts 2:38). Change of behavior does not earn forgiveness, but change of behavior demonstrates a genuine change of mind - from believing that one is righteous to believing that one is sinful and in need of salvation - that results in forgiveness. Neither does baptism result in forgiveness of sins, as some advocate. Rather, baptism demonstrated true repentance, which led to the forgiveness of sins.