God gives every man a conscience, and natural law exists for those who have not yet been brought under His Word.
Romans 2:14 For when the Gentiles, who have not the law, do by nature those things that are of the law; these having not the law are a law to themselves:
It is a dangerous misconception that scripture is the only authoritative source of truth about God:
Romans 1:20 For the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity: so that they are inexcusable.