Reasoning involves using the laws of logic. These laws include the law of non-contradiction that says you cannot have A and not-A at the same time and in the same relationship. Example: "My keys are in the house, and it is not the case that my keys are in the house" is necessarily false by the law of non-contradiction. As a Christian I have an absolute standard for reasoning; I am to pattern my thoughts after God's thoughts and the laws of logic are a reflection of the way God thinks. The law of non-contradiction is not just an opinion of how we ought to think, but rather it stems from God's self-consistent nature. God cannot deny himself (2 Timothy 2:13), and so, the way God upholds the universe will be n0n-contradictory.
Laws of logic are God’s standard for thinking. Since God is an unchanging, sovereign,immaterial Being, the laws of logic are abstract, universal, invariant entities. In other words,they are not made of matter—they apply everywhere and at all times. Laws of logic are
contingent upon God’s unchanging nature. And they are necessary for logical reasoning.Thus, rational reasoning would be impossible without the biblical God. (Dr. Jason Lisle)
What if God doesn't exist?
Or, from another angle; why would God give us the capacity to reason to the point that we could reason that God did not exist? Doesn't this refute the quote from 2 Timothy you gave? If God cannot deny himself then shouldn't it be impossible for someone to use their reason to conclude that God does not exist? Wouldn't this be God denying himself through human reason?
You seem to be under the impression that your own logical conclusion of the existence of God is, and should be, everyone else's logical conclusion. The fact that there are atheists who have logically concluded there is no God easily refutes this assumption. If you have reasoned that there is a God, I am happy for you. I have spent years trying to reason with the Bible and eventually concluded that there is no God. Let's take a little sidetrack and I will show you one thing that made me conclude, using my reasoning, that there is no God;
In Genesis 1:11 God created all the plants and vegetation on earth. This was the third day of creation. However, there is a problem with this. There is no sun to provide light for these plants to live. The sun isn't created until the next day. So the plants had to survive for a whole day without the sun's light. Now, I realise Christians will point to the "light" that God created on the first day of creation. But this light is unspecified. What is it? Where does it come from? It is
not the light from the sun, which plants need to live (look up photosynthesis on wikipedia). The "light" on the first day is never mentioned again after the first day (as far as I know). The light was not from the person of God (or God would not have needed to have created it). It was clearly some kind of temporary light until God created the sun and moon and other stars.
From this rather bizarre story, and the creation story as a whole, I have to conclude that logically speaking, it didn't happen as described. I know, from studies in modern science, that plants need the sun in its current form to photosynthesise (
http://en.wikipedia.org/wiki/Photosynthesis). Therefore the story in the Bible is not true. In order for it to be true the onus falls on Christians to invent theories as to what they believe might have happened.
This is just one example of how I came to reason away the existence of God. A further example would be such things as the talking snake in the Garden of Eden. Snakes don't talk. This snake was
not the devil, because why would God punish a possessed snake (by making it crawl on its belly, which presumes the snake once had legs); it's just an animal. So therefore the story is explicitly suggesting that it was a literal talking snake that deceived Eve and not a spiritual being possessing the snake. Again, since talking snakes are not a regular occurrence I have to conclude that this story is therefore not true which leads me to doubt the integrity of the Bible as a whole.
Given these things I just don't see how one can conclude that reason is from God and therefore God must exist.