Michael Shermer describes his own disbelief in God as follows: “I once saw a bumper sticker that read ‘Militant Agnostic: I don’t know and You Don’t Either.’ This is my position on God’s existence: I don’t know and you don’t either.”
Shermer is right to call that position “militant”. To claim that nobody can know the truth of a proposition brings with it a high evidential bar. To be sure, there are some truth claims toward which I think one can readily defend a ‘militant’ agnosticism. For example, no human being can know the number of snowflakes that fell in the great New England blizzard of 1888. Estimates are that approximately 20,000,000,000,000 snowflakes fall in an average snowstorm. And no doubt, meteorologists can make far more precise estimates of the number of snowflakes that fell in the 1888 blizzard. But the actual number is inaccessible to human beings now.
As for knowledge as to whether God exists or not, that is a very different issue. That requires, first, that Shermer has sufficient familiarity with all the arguments for and against God’s existence to know that none of those arguments is sufficient to justify a belief that God does (or does not) exist. Second, Shermer must either have an irrefutable argument that no person could have knowledge of God apart from the subset of those arguments that support theism (i.e. that argument must show that such belief could not be justified based on direct personal experience); conversely, he must have special access to all ranges of non-discursive possibly justified belief about God to know that none of those possible ranges of non-discursive belief occasioned by experience could possibly justify said belief.
Needless to say, Shermer has none of this. All he has is a wholly unjustified declaration that nobody can know the truth of a proposition about which he is agnostic.
 Michael Shermer, The Believing Brain: From Ghosts and Gods to Politics and Conspiracies—How We Construct Beliefs and Reinforce Them as Truths (New York: Henry Holt, 2011),175.