This I say therefore. — The phrase “This I say” seems to be used by St. Paul in returning (so to speak) from some lofty aspiration or profound reasoning, in which some might not be able to follow him, to a solid, practical ground, which all may tread. (See, for example, 1 Corinthians 15:50.) Here he is not content to use this phrase simply, but he enforces it by the solemnity of the adjuration “I testify” (comp. Acts 20:26; Galatians 5:3), which properly means, “I call God to witness the truth of what I say” — a phrase found in express terms in Romans 1:9; 2 Corinthians 1:23; Philippians 1:8; 1 Thessalonians 2:5. Nor was even this enough, for he adds “in the Lord” — that is, in the name, authority, and spirit of the Lord Jesus Christ Himself. The whole form is therefore one of peculiar force and solemnity.
The vanity of their mind. — In these words St. Paul describes the fundamental condition of heathenism. The “mind,” that is (as in Romans 7:23; Romans 7:25), the “inner man” — the spiritual intuition of invisible principles of truth and right, which is the true humanity — has become “subject to vanity” (Romans 8:20), — the vanity of which the Book of Ecclesiastes so often speaks. In losing the living conception of a living God, it has lost also the conception of the true object and perfection of human life; and so wanders on aimless, hopeless, reckless, as in a dream. With what absolute fidelity St. Paul describes the heathen world of his day, its history and its literature alike testify. Compare with the whole passage the picture drawn in Romans 1:21-32, “They became vain in their imaginations, and their foolish heart was darkened,” &c. The difference is that in the latter passage the prominent idea is mainly of “judicial blindness,” sent by God as a penalty on wilful apostasy from Him, whereas here St. Paul rather dwells on self-chosen blindness and hardness of heart.