We can never be certain as to God's existence
Knowledge of God’s existence is unattainable because no evidence could ever justify religious belief
David Hume, the Scottish philosopher, contended that meaningful statements about the universe are always qualified by some degree of doubt. He asserted that the fallibility of human beings means that they cannot obtain absolute certainty except in trivial cases where a statement is true by definition (e.g. tautologies such as "all bachelors are unmarried" or "all triangles have three corners"). We cannot, therefore, make definitive statements about the existence or non-existence of God.
Many describe themselves as agnostics because they believe that certain knowledge is impossible in religious matters. Whatever evidence there might be for or against the existence of God, both God’s existence and God’s non-existence remain conceivable. If we form a belief on uncertain evidence then we might turn out to be wrong. It is therefore better for us to withhold our judgement, to remain agnostic.
This argument is not especially persuasive. Though it is plausible to think that we cannot ever attain certainty as to whether or not God exists, this is true of all matters; nothing can be proved beyond all doubt. Descartes‘ argument from error establishes this: I have made errors of reasoning in the past, even concerning simple matters, and so can on no occasion be certain that I am not in error again. In spite of this unavoidable uncertainty, we nevertheless form beliefs. Why should we not do the same in matters of religion?
A more subtle version of the argument from uncertainty has an answer to this question: because religion is so important. Religion matters, and that is why we ought to be particularly careful in forming our religious beliefs.
The more important it is to be right about a matter, the more cautious we should be in forming our beliefs. If a matter is of great importance, as religion is, then our evidential standards concerning it should be set high, we should demand strong evidence before settling on what we believe.
In fact, religion is of unquantifiable importance - ”there is nothing more important than being right about the question of God’s existence” - and we should therefore set our evidential standards infinitely high.
If this is correct, then the standard of evidence required for justified religious belief is so high that it can never be satisfied; we can never have enough evidence to form beliefs about such questions as whether God exists. In this way, the importance of religion works to suggest that we can never have religious knowledge, that we ought to remain agnostic.