God is a complex abstract that is difficult to explain without the ontological argument.
For God to exist, God must be “a being than which nothing greater can be thought” (God I, 9). This statement is really the essence of the ontological argument; this specifies that if there is a greater being than God, then God does not exist. The modus tollens is constructed in the way such that if a contradiction follows using the true premises, from the supposition, then the supposition must be false (God I, 12). Using the reductio, Anselm proved that there is no higher, superior being that God; therefore God must exist.