There are two things going on here.
- Nobody can prove that something does not exist.
- The default conclusion is not that because something can not be proven that it must be true. It is also not natural that just because someone believes in this mysterious, invisible, undetectable thing that their beliefs must therefore be respected.
To better illustrate this, I turn to a very wise man named Bertrand Russell. The following are his words from Illustrated magazine in 1952.
"Many orthodox people speak as though it were the business of sceptics to disprove received dogmas rather than of dogmatists to prove them. This is, of course, a mistake. If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time."
A more modern version of this theory is illustrated in the Church of the Flying Spaghetti Monster. The followers of this "religion" believe that to be blessed it to be "touched by His noodley appendage". "Pastafarians", as they call themselves, even have their own website here and have published their first book of scripture.
Since it is impossible for anyone to prove that the Flying Spaghetti Monster does not exist, it should therefore be, in a Christian's mind, unnatural, immoral and unforgivable for them not to respect a Pastafarian's beliefs.
Sorry, Christians, but if you want us to believe in God, you had better pray that God shows himself. The evidence against the existence of God is profound and there is zero evidence that God exists at all. You want us to join your congregation and polish the heels of your already well-heeled pastor, just because you say there is an invisible god-thing peering at us through a very powerful and very large surveillance camera who will punish us if we don't just take your word for it?