Although it is true that no one can forgive sins but God alone, our Lord bestowed this ability – through His Name – upon the Apostles, and in turn, those to whom the Apostles ordained and laid hands on.
In John’s gospel, when the Apostles were gathered together after the Resurrection, Jesus suddenly appeared to them and said: “Peace be unto you. Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained.” (John 20:23)
Each Orthodox priest, by virtue of his ordination, becomes a member of the apostolic succession through the laying on of hands. That is to say, Jesus ordained the apostles, the apostles ordained others, and those others ordained still others and so on down through the generations of human history to modern times. Therefore each Orthodox priest traces his ordination back through an unbroken chain which leads directly to the apostles, and thus – to Christ Himself. This is the justification for the clergy to forgive sins within the Sacrament of Confession.