St. Thomas Aquinas said:
1. The "unmoved mover" argument. We know that there is motion in the world; whatever is in motion is moved by another thing; this other thing also must be moved by something; to avoid an infinite regression, we must posit a "first mover," which is God.
2. The "nothing is caused by itself" argument. For example, a table is brought into being by a carpenter, who is caused by his parents. Again, we cannot go on to infinity, so there must be a first cause, which is God.
3. The cosmological argument. All physical things, even mountains, boulders, and rivers, come into being and go out of existence, no matter how long they last. Therefore, since time is infinite, there must be some time at which none of these things existed. But if there were nothing at that point in time, how could there be anything at all now, since nothing cannot cause anything? Thus, there must always have been at least one necessary thing that is eternal, which is God.
4. Objects in the world have differing degrees of qualities such as goodness. But speaking of more or less goodness makes sense only by comparison with what is the maximum goodness, which is God.
5. The teleological argument (argument from design). Things in the world move toward goals, just as the arrow does not move toward its goal except by the archer's directing it. Thus, there must be an intelligent designer who directs all things to their goals, and this is God.
When in doubt, always choose the theory that seems most reasonable. 
