1:1-7 ... Salutation. Ancient Greek letters customarily began with the names of the sender & the recipient & a short greeting. Paul expands the usual form to express his Christian faith as well. God's Son, who came into the world physically descended from David, was manifested & installed in his true status at the resurrection. Saints, those who belong to God, consecrated to his service. Grace ... & peace.
1:8-15 ... Thanksgiving. After the salutation in ancient letters there usually came a short prayer of thanksgiving or of petition on behalf of the person addressed. This element also Paul expands in a characteristically Christian way.
Reference summary used from The New Oxford Annotated Bible with The Apocrypha Expanded Edition, An Ecumenical Study Bible, RSV