In other words, if you sit and think about who God is supposed to be, you realize that such a being is impossible. Ridiculous, in fact.
If you consult the dictionary, here is the first definition of God that you will find:
    "A being conceived as the perfect, omnipotent, omniscient originator and ruler of the universe, the principal object of faith and worship in monotheistic religions." 
Most believers would agree with this definition because they share a remarkably clear and consistent view of God. Yes, there are thousands of minor quibbles about religion. Believers express those quibbles in dozens of denominations -- Presbyterians, Lutherans, Catholics, Baptists, Episcopalians, Methodists and such. But at the heart of it all, the belief in God aligns on a set of core ideas that everyone accepts. What if you were to simply think about what it would mean if there were a perfect, omnipotent, omniscient originator and ruler of the universe? Is it possible for such a being to exist? Epicures thought about it in 300 BCE, and he came up with this:
    "The gods can either take away evil from the world and will not, or, being willing to do so, cannot; or they neither can nor will, or lastly, they are both able and willing. If they have the will to remove evil and cannot, then they are not omnipotent. If they can, but will not, than they are not benevolent. If they are neither able nor willing, then they are neither omnipotent nor benevolent. Lastly, if they are both able and willing to annihilate evil, how does it exist?"
Take this quote from the Bible. In Matthew 7:7 Jesus says:
    Ask, and it will be given you; seek, and you will find; knock, and it will be opened to you. For every one who asks receives, and he who seeks finds, and to him who knocks it will be opened. Or what man of you, if his son asks him for bread, will give him a stone? Or if he asks for a fish, will give him a serpent? If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!
The impossibility of God is visible here as well. Based on Jesus' statement, let's assume that you are a child and you are starving in Ethiopia. You pray for food. What would you expect to happen based on Jesus' statement? If God exists as an all-loving, all-knowing and all-powerful parent -- a "father in heaven" -- you would expect God to deliver food to you. In fact, the child should not have to pray. Normal parents provide food to their children without their children having to beg for it. Yet, strangely, on planet Earth today we find tens of millions of people dying of starvation every year. Another way to approach the impossibility of God is to think about the concept of omniscience. If God is omniscient, then it means that he knows every single thing that happens in the universe, both now and infinitely into the future. Do you have free will in such a universe? Clearly not. God knows everything that will happen to you. Therefore, the instant you were created, God knows whether you are going to heaven or hell. To create someone knowing that that person will be damned to hell for eternity is the epitome of evil.
Here is another way to understand the impossibility of God. If you look at the definition of God, you can see that he is defined as the "originator and ruler of the universe". Why does the universe need an originator -- a creator? Because, according to religious logic, the universe cannot exist unless it has a creator. A believer will say, "nothing can exist unless it is created." However, that satement immediately constructs a contradiction, because we must then wonder who created God. For a believer the answer to that is simple -- "God is the one thing that does not need a creator. God is timeless and has always existed." How can it be that the everything MUST have a creator, while God must NOT? The contradiction in the definition of God is palpable.
As soon as your think about the concept of a perfect, omnipotent, omniscient being, you realize the impossibility of the concept. That impossibility is yet another way to see that God is imaginary.