Useful Quote: Existance of God

In Romans 1:18 ff we read, "For the wrath of God is revealed from heaven against all ungodliness and righteousness of men , who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made.  So they are without excuse."

The quote I found useful regarding this idea of knowing God comes from Calvin's Institutes regarding the existence of God, Chapter III:

"There is within the human mind, and indeed by natural instinct an awareness of divinity.  This we take to be beyond controversy. To prevent anyone from taking refuge in the pretense of ignorance, God himself has implanted in all men a certain understanding of his divine majesty... If ignorance of God is to be looked for anywhere, surely one is most likely to find an example of it among the more backward folk and those more remote from civilization. Yet, there is, as the imminent pagan says, no nation so barbarous, no people so savage, that they have not a deep-seated conviction that there is a God."

Footnote in the Institutes: The pagan referred to above is Cicero.  "Calvin's view that all men have a natural sense or intimation of deity is in accord with the presupposition of all the characters of Cicero's dialogue On the Nature of the Gods, including the Epicurean, Velleius, who asks: 'Where is there to be found a race or tribe of men which does not hold, without instruction, some preconception of the gods?'" Nature of the Gods I. xvi.43