Paul uses this term because they are his fellow-Jews and followers of Judaism. They are not Christian believers at this point. Alternate translation: "my fellow Israelites and other friends"
This can be stated in active form. Alternate translation: "that we proclaim to you that your sins can be forgiven through Jesus" (See: [[rc://en/ta/man/translate/figs-activepassive]])
The abstract noun "forgiveness" can be translated with the verb "to forgive." Alternate translation: "that God can forgive your sins" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])