Agnostic theism isn't a term that is frequently used by theists themselves, but the concept is not unheard of — especially among mystics. Gregory of Nyssa, for example, insisted that God was so transcendent that God must necessarily be forever unknown and unknowable.
Agnostic theism can also be defined a bit more narrowly as belief in the existence of a god but not knowing the true nature or essence of this god. This definition of agnostic theism is a bit more common among theologians, some of whom accept it as reasonable and some of whom criticize it as insufficient.
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In colloquial use and indeed much traditional discussion, theists are those who believe that there is a God; atheists are those who believe that there is not; and agnostics are those who neither believe that there is nor believe that there’s not.
However, the etymology of ‘agnostic’ favours a deviation from colloquial use. We might say that agnostics are those who believe that they do not know whether or not there’s a God; they may nevertheless believe that there is or believe that there’s not. On this understanding of agnostic then, it is quite possible for theists or atheists to be agnostics.
An agnostic theist, for example, would believe that there is a God but also think that his or her belief that there’s a God did not have whatever it is that must be added to true belief to make it knowledge.
- T. J. Mawson, Belief in God An Introduction to the Philosophy of Religion