Based on the principle of pre-established harmony and the theory of the law of sufficient reason, Leibniz built his cosmological argument from a different perspective and completed his theodicy accordingly. Leibniz's theological thought enunciated the relationship between the almighty God and vices of all kinds, reaching the optimistic conclusion that our world is the best among all possible worlds. This essay tries to probe into Leibniz's theology and lays bare his effort to cover all contradictions underlying universal harmony, which is a striking character of his theology.