Can the faithful on earth help one another? The faithful on earth, as members of the Mystical Body of Christ, can help one another by practicing supernatural charity and the spiritual and corporal works of mercy.
Pray for one another that you may be saved. For the unceasing prayer of a just man is of great avail. (James 5:16)
What is meant in the Apostles' Creed by "the forgiveness of sins"? By "the forgiveness of sins" in the Apostles' Creed is meant that God has given to the Church, through Jesus Christ, the power to forgive sins, no matter how great or how many they are, if sinners truly repent.
Receive the Holy Ghost; whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained. (John 20:22-23)
Further reading: CCC 976