St Thomas theodicy is based on “five ways” which prove the existence of God:
The way of motion: There are things in our world which are moved. Buteverything that is moved, is moved by another. But, for that, there should be something or someone who isn't moved by anyone. This one is God.
The way of causation: In theworld of sensible things, there's an order of efficient causes. There is no case known in which a thing is found to be the efficient cause of itself and it isn't possible togo on to infinity, so, it is necessary to admit a first efficient cause: God.
The way of Contingency: Everything in the world may or may not exist. We know fromexperience that all things change in one way or another. But that which may not exist, does not have the reason of its existence in itself, but in another, that is, in somethingwhich may exist. So there exists the necessary being, which is God.
The way of Goodness: We found, in this world, things more and less good, true, noble, and the like.Because of that, there should be the most being, for those things that are greatest in truth are greatest in being... Now the maximum in any genus is the cause of all in thatgenus, as fire, which is the maximum of heat, is the cause of all hot things... Therefore, there must also be something which is to all beings the cause of their being, goodness,and every other perfection: God.
The way of Design: We see some things which lack knowledge, natural bodies, which acts for an end. Things without knowledge doesn't actsfor an end if they aren't being directed by some being endowed with knowledge and intelligence. So there's an intelligent being that direct all natural things for an end: God