Adam Clarke on Romans 1:9

Whom I serve with my spirit - λατπευω Whom I worship with the profoundest religious reverence; for so the original certainly means: I not only employ all the powers of my body in this service, but all those of my soul; being thoroughly convinced of the absolute truth of the religion I preach. Probably St. Paul opposes, in this place, the spiritual worship of the Gospel to the external, or what some call the carnal, worship of the Jews. Mine is not a religion of ceremonies, but one in which the life and power of the eternal Spirit are acknowledged and experienced.