God is my witness; in these words there is the force, if not the form, of an oath. See the like, 2 Corinthians 1:18, 2 Corinthians 11:31 Galatians 1:20. His great love and care of them was a hidden thing, and known only to God; to him therefore he appeals for the truth thereof. Oaths, in certain cases, are allowable under the New Testament, as well as the Old. With my spirit, i.e. sincerely, or with my whole heart: see Ephesians 6:6 2 Timothy 1:3. Without ceasing, i.e. as often as he prayed. This was a great indication of his hearty affection to them.
Related Commentaries of Romans 1:9
Romans 1:9
9 For God is my witness, whom I serve withc my spirit in the gospel of his Son, that without ceasing I make mention of you always in my prayers;