The gospels of Mark and Luke both portray John the Baptist's ministry as being done "for the remission of sins":
John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins. (Mark 1:4)
And he came into all the country about Jordan, preaching the baptism of repentance for the remission of sins. (Luke 3:3)
On the other hand, Hebrews declares:
without shedding of blood is no remission. (Heb. 9:22)
Can the concept affirmed in Hebrews be reconciled with the teaching of Mark and Luke that, long before Jesus shed blood on the Cross, people could receive remission from sins through the baptism of John? If so how?
Related point: In Acts 18, Apollos became an important figure in the church. He had been baptized under John's authority and needed to be more fully instructed in "the Way." However he does not seem to have been required to undergo baptism again:
a certain Jew named Apollos, born at Alexandria, an eloquent man, and mighty in the scriptures, came to Ephesus. This man was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John. And he began to speak boldly in the synagogue: whom when Aquila and Priscilla had heard, they took him unto them, and expounded unto him the way of God more perfectly. And when he was disposed to pass into Achaia, the brethren wrote, exhorting the disciples to receive him: who, when he was come, helped them much which had believed through grace(Acts 18:24-27)