Evidence of the knowledge of God

My guess is that we have all received a request for prayer from friends and acquaintances who either do not normally recognize God in their lives or who are generally anti-God in their sentiments. Either trials or tribulations often precipitate this request for prayer. For Calvin, this points towards their knowledge of God's existence, despite their indifference or hostility. Calvin writes,

Nay, we have still stronger evidence of the proposition for which I now contend—viz. that a sense of Deity is naturally engraven on the human heart, in the fact, that the very reprobate are forced to acknowledge it. When at their ease, they can jest about God, and talk pertly and loquaciously in disparagement of his power; but should despair, from any cause, overtake them, it will stimulate them to seek him, and dictate ejaculatory prayers, proving that they were not entirely ignorant of God, but had perversely suppressed feelings which ought to have been earlier manifested. (Institutes, 1.4.4)