Jamieson, Fausset and Brown on Romans 1:9

9. For God ... whom I serve--the word denotes religious service.

with my spirit--from my inmost soul.

in the gospel of his Son--to which Paul's whole religious life and official activity were consecrated.

is my witness, that without ceasing I make mention of you always in my prayers--so for the Ephesians (Ep 1:15, 16); so for the Philippians (Php 1:3, 4); so for the Colossians (Col 1:3, 4); so for the Thessalonians (1Th 1:2, 3). What catholic love, what all-absorbing spirituality, what impassioned devotion to the glory of Christ among men!