19 Because that which may be known of God is manifest in them; for God hath shewed it unto them. (Romans 1:19 KJV)
William Burkitt’s Commentary
That is, much of the nature and properties of God may be known by the light of nature; his infinite power, wisdom and goodness, are manifest in the minds and the consciences of all men; For God hath shewed it unto them, partly by imprinting these notions of himself upon the hearts of all men, and partly by the book of the creatures in which his glorious attributes are written in large and legible characters.
Learn hence, That all men have a natural knowledge of God, and those great duties which result form the knowledge of him.
2. That the natural knowledge which men have of God, if they live contrary to it, is sufficient evidence of their holding the truth of God in unrighteousness and is a God-provoking and wrath-procuring sin.