Not to be confused with simony.
Parsimony, or the "principle of parsimony", is a method for selecting between equally valid hypotheses based on the complexity of each. Using the principle of parsimony the simplest hypothesis, or the one with the fewest free parameters, will be preferred over more complex hypotheses if other selection criteria are equal between them.
 Parsimony and God
Scientific and tangible evidence for the existence of God is either weak and non-compelling, or completely non-existent (depending on how much credit you give to personal experience, or holy texts). Therefore, with the existence and non-existence of God as hypotheses being on roughly equal footing with respect to the complete lack of physical evidence, the principle of parsimony may be applied to them. Theistic scholars sometimes pre-assume the existence of God because of this principle. This is based on the idea that God's existence and its omnipotence creates a simple solution of the origin of the universe and life in comparison to the far more complex conjectures involved in cosmology.
Rationalists and atheistic scholars generally reject this conclusion on the basis that God, whatever it is, must be more complex than the universe itself in order to create and control it in an omnipotent manner. God must hold within it the information contained within the universe in addition to its past, future and potential states, and, on top of all of this, information about "how to be God". This results in an incredibly complex hypothesis to pre-assume, and should therefore be rejected on the grounds of parsimony.