Ask them to think about the total sum of universal knowledge ... the sum total of everything that is knowable. Then ask them what percent of that total knowledge do they possess.
Even the most arrogant atheist can be slapped silly by making them recite the atomic number of Fermium, or the most efficient way to solve the Magnetic Field Integral Equations, etc.
At this point, the reasonable ones would be willing to admit that they possess less than 1% of all that is knowable (which in itself is a gross exaggeration).
At that point your question is simply "why do you assume that God cannot be part of the 99% that you admit you do not know?"
Ignorance is not the problem; belief in things not true is. What makes you think believing in god(s) is not belief in things not true?