"The heavens are telling the glory of God; and the firmament proclaims his handiwork. Day to day pours forth speech, and night to night declares knowledge."
Psalm 19:1-2 (RSV)
"For what can be known about God is plain to them because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the theings that have been made."
Romans 1: 19-20 (RSV)
"For, in the first place, no man can survey himself without forthwith turning his thoughts toward the God in whom he lives and moves; because it is perfectly obvious, that the endowments which we possess cannot possibly be from ourselves; nay, that our very being is nothing else than subsistence in God alone."
-- John Calvin, Institutes of the Christian Religion, p. 1.
All these quotes make the clear point that knowledge of God's existence and basic attributes is manifestly possible and reasonable. However, possessing of this knowledge does not imply that it is attained by a reasoning process. It is more like a tacit assumption. And since God is by definition the Creator, it is a starting-point assumption.