I fear this will turn into a definitional debate, but nonetheless, as I am the affirmative, I shall supply a definition of the topic.
That there is - currently in existence, but due to the nature of a monotheistic deity, always has been and will be in existence.
a monotheistic deity - a single god. A deity is defined as a supreme and absolute power, higher than humans with none higher, superior, worthy of worship, omniscient, omnipotent and omnipresent, perfect, uncreated, without beginning or end.
There are multiple positions relating to the existence of a deity or deities. Those contrary to monotheism are atheism, pantheism, and polytheism. I shall address each in turn.
Atheism - the lack of belief in the existence of God or gods.
I am going to subscribe to some more Jewish philosophy here and simply offer the existence of the world as complete and undeniable proof of the existence of a deity. You tell me how something can come from nothing and how order can come from chaos (in direct defiance of scientific laws) and then I may give some credit to atheistic evolution. Without atheistic evolution, atheism is totally unviable, as I have found out very personally (for those who don't know, I am a former atheist).
Pantheism - the belief that God is everything and everything is God.
I offered the universe as evidence disproving atheism, and I'll offer myself and my computer desk as evidence disproving pantheism. I'm going to overlook the obvious disproving evidence that a deity is higher and superior than humans by default and point out the fact that a deity is perfect and omniscient. Am I perfect and omniscient? I'd like to think I am, but even I will confess to the fact I am not. Furthermore, if I was part of God, due to the inherent omniscient nature, I would already know this and so would you. I have heard that this knowledge simply needs to be unlocked, but God IS omniscient and that means he KNOWS everything (present tense), NOT that he will know anything or that some knowledge is currently locked from him.
Now what about my computer desk? Well, a deity is animate. It has life, the ability to think and reason, and the power to carry out absolute decisions. A desk has NONE of those qualities. My lovely computer desk holds my computer, my junk, and nothing else. It's got no thought capabilities. It cannot be God because it is INANIMATE. I am not God, nor am I typing on God. I think it is fair to say that the existence of myself and my computer desk disproves pantheism.
One other point I would like to make is that a deity is not created. It may have chosen to make creation in its image depending upon your theology, but it is an uncreated, eternal being. The world is created and temporary. Therefore, everything is not God.
Polytheism - the belief in multiple gods.
This is potentially the hardest to argue against on the surface. Atheism and pantheism have the universe and myself to try to explain away, but off the top of my head, I cannot think of a single similarly easy piece of evidence to counter polytheism. So how do I know there aren't two gods or two thousand and sixty? That's where a few arguments come in (and some credit must go to an Islamic theology website for actually inspiring portions of this).
A deity is not weak, inferior, or compromising. That's simply part of being a deity. I am sure you can find someone who'll say "My God's really weak, even I can tell him what to do, and he always has to compromise with my mother," but such a position is hardly rational or normal and it does not conform to the definition of a deity. So what does this logic mean? The existence of more than one god creates a number of contraidctory possibilities;
1. That one god is above another (or that god one is more powerful than god two who is more powerful than god three, et cetera). This means that at least one god is inferior and weak, negating their status of deity. A deity is perfect; you cannot have perfection above perfection. The situation whereby one god is above another is simply not possible.
2. That the gods are of equal power. This would mean they are compromising, and moreover, if they were in continual agreement, working, thinking, and acting as one, that would actually mean they are of one mind and one essence, creating - you guessed it! - monotheism.
3. That one god is wholly and completely good while another is wholly and completely evil. The nature of a deity is such that through their profound superior knowledge, they would comprehend and act upon virtues of love and compassion, practicing justice but not delighting in creating or causing evil, for that is opposite to their nature. This negates the existence of an evil deity. In addition, if the ultimately evil deity did exist, there are two possibilities.
One is that it would be of equal power to the good deity, and therefore both good and evil would be negated and neutrality would reign. Look around you. Does neutrality reign? No. Furthermore, if either or both god is negated out or has their power cancelled out by the opponent, it's not a deity and a deity is superior and able to exercise its will.
Alternatively, either the good or evil deity could be superior. If the evil deity were superior, it would vanquish the good deity. If the good deity were superior, it would either vanquish evil or practice mercy, but as shown above, neither an inferior or an evil deity could possibly exist.
I think I shall leave it there and open things up to you, the reader now. I feel I have sufficiently proved my points against atheism, pantheism, and polytheism, leaving monotheism as the only option left to select. It is logical - existence necessitates a deity and logic necessitates that it is not pantheistic or that multiple deities exist. Is there a monotheistic deity? Most definitely.
(If you hold to an alternate position or would like me to address another position you know of, then go ahead and make a comment. If you'd like to discuss it yourself for the benefit of others, feel free to do so, and if you'd like to do that on your journal, at least tell me so I can link to your post.)
Considering the success and failure of this debate and interest in another, I am contemplating holding another debate on a topic along the lines of 'Who is the correct monotheistic deity?' That's just a thought I had, though. For now, join in on this one!