In The Varieties of Religious Experience (1902), in Lecture XVIII on Philosophy, William James says:
“If you have a God already whom you believe in, these [philosophical arguments for God’s existence] confirm you. If you are atheistic, they fail to set you right. The proofs are various. The “cosmological” one, so-called, reasons from the contingence of the world to a First Cause which must contain whatever perfections the world itself contains. […]
The fact is that these arguments do but follow the combined suggestions of fact and feeling. They prove nothing rigorously. They only corroborate our preëxistent partialities.” (p. 476)
I think much the same can be said of atheism – often the intuition (a conglomeration of various facts and feelings) that there is no “God” (or, a lack of an intuition that there is a God) comes first, the scientific or philosophical reasons or arguments are consequent – that is, they corroborate preëxisting partialities.
This topic moves into coherence theory and the nature of evidence.
(Also see here.)