“…they were being baptized by him in the Jordan River, confessing their sins.” (Mark 1:5b)
John the Baptist was preaching a baptism of repentance for the forgiveness of sins (Mk 1:4). This preaching had to begin with a proclamation of God’s Law, for only the Law causes us to realize our sins. Thus from John’s preaching of the Law the people wanted forgiveness, a forgiveness given in this God-ordained pre-Sacrament—the baptism of John. John’s preaching and baptism involved repentance, and thus the people were expected to acknowledge and turn from their sins. The acknowledgement of their sins was not simply an acknowledgement that they were sinners; no, it went further than that. When an individual stood before John, they were ready to be baptized by him in the Jordan River, confessing their sins. It is apparent from this wording that at least John audibly heard the confession of specific sins; tax collectors confessed cheating, soldiers confessed abusing their authority, lustful men and women confessed this sin, and so on. Quite clearly the text is telling us that this was not a general confession of sinfulness, but it was a confession of actual sins…sins of omission or sins of commission.
Many of us in our American culture have lost sight of such confession of specific sins spoken out loud, but the Jews were quite familiar with it. The Jewish scholar Maimonides (b. 1135) conveyed some of the ancient Temple practices passed down for a millennia. In the OT sacrificial liturgy, before a worshiper would slay his burnt/sin/guilt offering he would lay his hand(s) upon the creature’s head. While in this posture—as explained by Maimonides—the worshiper would privately confess his sins in the presence of the officiating priest. Sins were specified, out loud. The ancient liturgical “formula” for such private confession was: I entreat, O Jehovah: I have sinned, I have done perversely, I have rebelled, I have committed (naming the sin, trespass, or, in the case of a burnt-offering, the breach of positive or negative command); but I return in repentance, and let this be for my atonement (covering). It should then come as no surprise that those being baptized by John the Baptist recognized his authority simply from his priestly office. John was indeed a priest, having been born into the Jewish priestly order through his father, Zechariah, who was a priest from the priestly division of Abijah (Lu 1:5). It made a degree of sense to come and confess sins before this priest, John the Baptist.
But where then was the sacrificial creature upon whose head such sins were being placed? Our Lord’s baptism by John is sandwiched between John’s double-declaration that Jesus is the Lamb of God who takes away the sin of the world (Jn 1:29,36). The church through the centuries has recognized Jesus’ baptism to be the time when the sins of the world were laid upon Him. At our Lord’s baptism, the great reversal was occurring: All sins were being placed upon the Lamb of God, and His righteousness was then for sinners. Jesus was thus fulfilling all righteousness (Mt 3:15). At the cross the Lamb would be sacrificed, and He would be raised again for our justification.
Though the Christian Sacrament of Holy Baptism far transcends that of John’s Baptism—because Christian Baptism directly links us to Him and to all the blessings flowing from His cross and empty tomb—yet there is the parallel gift of the forgiveness of sins. Such forgiveness is indeed conveyed through Holy Baptism, and it continues to be offered as we sinners confess our sins—yes specific sins can be confessed aloud before a pastor—and the pastor privately pronounces the forgiveness of those sins in Jesus’ name. We thus see the consistent pattern flowing from the Old Testament, through John’s baptism and now in New Testament times: Sins are confessed, and forgiveness is pronounced because of the Lamb of God who takes away the sin of the world.
 Edersheim, The Temple…, 82. Parenthetic comments are in the text.