Romans
King James Version (KJV)

“For God is my witness, whom I serve with my spirit in the gospel of his Son, that without ceasing I make mention of you always in my prayers;”
King James Version (KJV)
1:9 God, whom I serve - As an apostle. In my spirit - Not only with my body, but with my inmost soul. In the gospel - By preaching it.
Ro 1:9 For. This introduces a reason for his thankfulness. He feels so deep an interest in the cause of Christ in that great center that he prays for them daily. Serve with my spirit. Not an outward service, but of the whole heart, soul and life, in the gospel to which he is consecrated. The spirit is the highest part of a man's nature, and service with the spirit is the highest service. Make mention of you always. So, too, of the Ephesians, Philippians, Colossians and Thessalonians. See Eph 1:15,16 Php 1:3 Col 1:3 1Th 1:2. Such statements show the deep solicitude of the apostle for the saints.