It means that atheist don't know that everything including the truth, reality and physicality is the result of and originated in and with God's Mind.
Atheists certainly do not know this, but then you do not know this either. You believe this, and think that your belief is justified and true and constitutes knowledge. But it is yet to be shown that your belief actually is true, or indeed that it is justified. You feel it is justified, but atheists do not see the evidence such as it is, as reaching the threshold for justifying such a belief. It may be that you have access to thoughts, dreams and experiences that the atheist lacks. Or it may be that you are imagining things.
The agnostic would say that we do not know these things, so the belief in God is unjustified. The atheist would say, in addition, that this is not just an unjustified belief, but I believe that it cannot be true either. I refuse to hold a belief for which I see no justification and which I think cannot be true. There is nothing unreasonable about any of the positions on God. People come to the concept of God from different start points and with different interpretations of the information available, which is often nebulous and intangible. It is inevitable that a range of conclusions will be reached.