In "Word Focus" Glenn McClure writes:
The word for service is a religious word that means “service'' or “worship.” It is used five times in the New Testament, where it always refers to religious service, not secular. Thus, true reasonable service is literally a spiritual function (not an external function) whereby man's spirit is in communion with God's Spirit. Man's spirit is that part of his personality which reflects its closest and most intimate relationship. From such a fellowship with God, man's spirit not only reflects that Christlikeness which results from the association, but also responds in obedience to the will of Him who has justified his spirit.
Source: Studies in Romans: Adult Teacher's Insights, page 64.