For God is my witness, whom I serve in my spirit in the gospel of his Son, that without ceasing I make mention of you in my prayers. A like solemn asseveration is made with a like intention (Philippians 1:8; cf.
also 2 Corinthians 11:31). It expresses the writer's earnestness, and is in place for attestation of a fact known only to himself and God. The word λατρεύω, ("I serve"), when used in a religious sense, most usually denotes "worship," and specifically the priestly services of the temple (Hebrews 8:5; Hebrews 9:9; Hebrews 10:2; Hebrews 13:10).
St. Paul's λατρεία intended here is not ceremonial function, but a spiritual one ( ἐν τῷ πνεύματί μου)—an inward devotion of himself to God's service in proclaiming and furthering "the gospel of his Son."
A similar view of the essential λατρεία of Christians is found in Romans 12:1; Romans 15:16; Philippians 3:3; 2 Timothy 1:3; Hebrews 9:14.