Tim Keller had this to say about the relationship between immorality and irrationality:
Every one of our sinful actions has a suicidal power on the faculties that put that action forth. When you sin with the mind, that sin shrivels the rationality. When you sin with the heart or the emotions, that sin shrivels the emotions. When you sin with the will, that sin destroys and dissolves your willpower and your self-control. Sin is the suicidal action of the self against itself. Sin destroys freedom because sin is an enslaving power.
In other words, sin has a powerful effect in which your own freedom, your freedom to want the good, to will the good, and to think or understand the good, is all being undermined. By sin, you are more and more losing your freedom. Sin undermines your mind, it undermines your emotions, and it undermines your will.[1]
See also What I’ve Been Reading: The Making of an Atheist, Part I
HT: STR
[1] Available from http://www.desiringgod.org/blog/posts/fighting-sin-with-worship. Excerpt from a sermon available from http://sermons2.redeemer.com/sermons/sin-slavery.
Thinking to Believe 
August 15, 2012 at 7:31 am
This makes a LOT of sense!
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August 15, 2012 at 7:32 am
No wonder atheists are so irrational.
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August 16, 2012 at 5:52 am
Very interesting idea. Especially to a believer…
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August 16, 2012 at 8:25 am
Doesn’t seem like a great plan for God to set humans up to be more likely to fail after a few missteps. Does he want us all in hell or is this Satan’s work?
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August 16, 2012 at 11:07 am
Grundy, no one said how much sin one must engage in before they destroy themselves spiritually. From experience, however, I think I can say it usually requires a lot of sin.
Jason
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August 18, 2012 at 1:07 am
This speaks to the “deceivableness of unrighteousness” as mentioned in 2 Thessalonians 2:10 and the “deceitfulness of sin” mentioned in Hebrews 3:13 (both deceivableness and deceitfulness are the same Greek words).
If anyone has read The Sin of Lying by B. E. Echols, he mentions several times how lying causes someone to dote, i.e. become a doddering fool whose mind is severely hampered, i.e. lost of all reason. His argument is based on Jeremiah 50:36, where the word dote is ya’al in Hebrew and has this meaning:
to be foolish, become fools, act foolishly, show wicked folly
So, in this context, I agree with the assessment given: that sin ravages one’s ability to think clearly and make wise decisions.
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