Arguments for the existence of God involve carefully crafted reasoning with the hope that an individual will come to the conclusion that God exists.
Also known as proofs for God's existence, these arguments have not always come with full acceptance. Those opposed to natural theology claim that God's existence cannot be proven by human reason or the natural world, and that any attempt to do so runs the risk of becoming a God of the Gaps argument. Viewpoints vary, but responses tend to conclude that God can only be known by supernatural revelation or Scripture alone. Karl Barth is a classic example of this as he believed that God is exclusively revealed in Jesus Christ, and Jesus is only revealed in the Bible. Proponents of natural theology vary as well, but most conclude that the existence of God can be known through human reason although it is not salvific (not a saving knowledge of God). Thomas Aquinas is characteristic of this view, holding to the understanding that the created world reflects aspects of its creator that are apparent to all. Nonetheless, arguments for the existence of God have been formed throughout church history and continue to be used today, namely in the area of apologetics.
Kalam cosmological argument
The aim of this argument is to show that the universe had a beginning in the finite past. The argument battles against the existence of an infinite, temporal regress of past events which implies a universe that has infinitely existed. This argument implies the existence of a First Cause.
The form of the argument is:
- Whatever begins to exist has a cause.
- The universe began to exist.
- Therefore, the universe has a cause.
Thomistic cosmological argument
- What we observe in this universe is contingent (i.e. dependent, or conditional)
- A sequence of causally related contingent things cannot be infinite
- The sequence of causally dependent contingent things must be finite
Conclusion: There must be a first cause in the sequence of contingent causes
Leibnizian cosmological argument
The argument comes from a German polymath, Gottfriend Wilhelm Leibniz. Leibniz wrote, "The first question which should rightly be asked is this: why is there something rather than nothing?"
The argument runs as follows:
- Every existing thing has an explanation of its existence, either in the necessity of its own nature or in an external cause.
- If the universe has an explanation of its existence, that explanation is God.
- The universe is an existing thing.
- Therefore the explanation of the universe is God.
Some atheists object to premise 2 in that God does not have to be the explanation, but that the universe can be what is called a necessary being (one which exists of its own nature and have no external cause). This was a suggestion of David Hume who demanded, "Why may not the material universe be the neccesarily existent being?" (Dialogues Concerning Natural Religion, part 9). The Kalam Cosmological Argument is helpful. If Hume (and other atheists) is right in saying that the universe is a necessary being/thing, then this implies that the universe is eternal. This is exactly what the Kalam argument seeks to disprove. Thus, the Kalam is a valuable supplement to the Leibnizian argument.
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The anthropological (anthropos meaning "man") argument is made on the basis of the condition of humanity, of mankind's basic moral standards and the thread a search for a higher being. It is related to the cosmological and teleological arguments in that it if man has a yearning for God and a conscience when offending him, ostensibly these have their origin and cause in God and not in man. The argument was perhaps most famously posited by Blaise Pascal, who reasoned that it was better "bet" to believe in God than not to do so.
"The ontological argument attempts to prove God's existence through abstract reasoning alone. The argument is entirely a priori, i.e. it involves no empirical evidence at all. Rather, the argument begins with an explication of the concept of God, and seeks to demonstrate that God exists on the basis of that concept alone.
"The argument is ingenious. It has the appearance of a linguistic trick, but it is a difficult task to say precisely what, if anything, is wrong with it. All forms of the argument make some association between three concepts: the concepts of God, of perfection, and of existence. Very roughly, they state that perfection is a part of the concept of God, and that perfection entails existence, and so that the concept of God entails God's existence." 
The ontological argument was first formulated by Anselm of Canterbury (1033-1109), one of the great medieval philosopher-theologians, in his Proslogium, Chapter 2. Anselm’s ontological argument rests on the identification of God as “that than which no greater can be conceived”. Once it is understood that God is that of which no greater can be conceived, Anselm suggests, it becomes evident that God must exist.
Descartes' ontological argument
We have the idea of an infinitely perfect Being. Since we are finite, and everything around us is finite, the idea of an infinitely perfect Being could not have originated with us or with the nature around us. Therefore the idea of an infinitely perfect Being must have come from such a being - God. 
Argument from desire
If and when we desire, we do so because what we desire exists prior to our desiring it. Religion at its basic level could be defined as a desire to please or appease a Supreme Being. Therefore this Supreme Being must exist.
"The Christian says, 'Creatures are not born with desires unless satisfaction for those desires exists. A baby feels hunger: well, there is such a thing as food. A duckling wants to swim: well, there is such a thing as water. Men feel sexual desire: well, there is such a thing as sex. If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world. If none of my earthly pleasures satisfy it, that does not prove that the universe is a fraud. Probably earthly pleasures were never meant to satisfy it, but only to arouse it, to suggest the real thing. If that is so, I must take care, on the one hand, never to despise, or be unthankful for, these earthly blessings, and on the other, never to mistake them for the something else of which they are only a kind of copy, or echo, or mirage. I must keep alive in myself the desire for my true country, which I shall not find until after death; I must never let it get snowed under or turned aside; I must make it the main object of life to press on to that other country and to help others do the same.'" (Lewis, 136-137)
- God, Are you There? Five Reasons God Exists and Three Reasons it Makes a Difference, by William Lane Craig (from the RZIM Critical Questions Booklet Series)
- Does God Exist? The Debate between Theists and Atheists, by J.P. Moreland (theist) and Kai Nielsen (atheist). Prometheus Books, 1993.
- Twenty Arguments For The Existence Of God, by Peter Kreeft & Ronald K. Tacelli
- Two Dozen (or so) Theistic Arguments by Alvin Plantinga; Lecture presented at the 33rd Annual Philosophy Converence, Wheaton College, Oct 23-25, 1986
- The Cosmological Argument: A Current Bibliographical Appraisal, by W. David Beck
- Evil as Evidence for God, by Justin Taylor
- The 4 Primary Arguments for God's Existence, by Michael J. Vlach
- 10 Arguments For God's Existence, by C Michael Patton