This may prove helpful for anyone engaging with unbelievers, particularly atheists who emphasize rationality and scientific evidence of a narrow sort.
The noetic effects of the Fall refer to mankind’s active suppression of their knowledge of God and his truth. As it goes, there is something internal that attests to God as well as something (many things) external that do the same. Internally, humans have consciences that tell them something of right and wrong. Would a human think it right to steal his neighbors goat unless someone taught him otherwise? Or, what about his wife? I don’t mean would they do it, but rather, that his conscience would have something to say about it.
Externally, knowledge of God comes by mere observation and reflection. The universe is magnificent, vast, and complex. We are small, not so vast, and in a uniquely miraculous way, complex. This tells humans something about things: there must be a Designer, and He must be very, very different from us.
Yet, the noetic effects of the Fall makes accepting the above like chewing gravel to the unregenerate. It’s best understood by reading Romans 1:18,
“For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth.”
Paul continues, saying that God’s nature is “plain to them” because he has “shown it to them” (1:19). He also writes that despite this plain knowledge, because of the rebellious suppression of it, that “they are without excuse. For although they knew God, they did not honor him as God… but they became futile in their thinking.” (1:20ff)
The noetic effects of the Fall cloud the unbelievers mind and prevent him/her from internalizing and believing in God’s truth or honoring him as God. It is only by the power of the Holy Spirit in response to the preaching of the Gospel of Jesus Christ that one’s soul is brought from death to life and into a saving knowledge of Christ. Then and only then, they are given the ability to honor God as God as a genuine response.