Thousands of religions have been claimed as real. You are your own custom version of a christian version. Your god, the one that you think is the true one, is just as invisible as all the others, which to you are obviously false, as they are to us. Your god requires faith and belief, just like all the false gods which you know are obviously not true. That is where we differ.
As we like to point out to all believers, you are an atheist when it comes to every other god but your own. So our atheism differs from yours by only one god. So close, and yet so far.
Most if not all of us consider all religions mythical. The Greeks used to actually believe in Zeus and Apollo and Atlas. The Romans believed in Janus and Mars and Neptune. The Norse had Odin, the American Indians had trickster coyotes and wise ravens. Each of these things was truly believed (and there are American Indians who still believe their stories today) but each is taught as myth in schools. Other religions older than your own have persisted, just as yours has. Hinduism too has holy books, holy men, holy holidays, holy expectations. The muslim religion is living proof that something can be made up on the spot and turned into a new religion. Yes it used some of christianity as a basis for its story, but it is also very different from your religion. And one guy whipped it out in just a few years.
Of course the lack of proof is relevant to atheists. The inaccuracy of "facts" within each religion makes them suspect as well. Your bible says that Jacob made striped goats by mating them in front of striped sticks. That is just plain silly, which makes it troublesome, being as the bible is, as we've been told by many, the perfect word of god. A real god would not include nonsense.
And if that were the only nonsense, we might be able to overlook it. But everything from the creation story and the ark to Moses running around for 40 yeaers in the desert with a million people and Jesus sacrificing himself for a few days adds to the nonsensical nature of the book. A god that sets up Adam and Eve to fail (which he knew they would because he is omniscient according to most) just so he could send down the kid to sacrifice himself is the stuff of myth and legend. But it is not the stuff of reality.
I have been an atheist for half a century and haven't once found my lack of belief challenged by anything I've encountered in reality. Nor have I encountered dirt-eating snakes, four legged insects or bats that were birds. I've not received a satisfactory explanation as to why your god was afraid of iron chariots or where the Egyptian gods that were not strong enough to ward off the christian god (the Moses staff/snake story) came from. Neither Egypt, which kept great records, nor Rome, which kept great records, made any mention of either Moses or Jesus. And christians everywhere have to find different ways to explain why JC didn't return in the lifetime of those he spoke to when he promised he would. I say different ways because one god and one kid have spawned well over 30,000 variations on the christian theme, where the religion hosts everything from snake bit snake handlers and very vocal tongue-speakers to dudes in popemobiles and tv evangelists. The only common thread being their request for money.
Now, toss into the mix science, which explains everything better than 2,000 year old sheepherders, and atheism trumps the living daylights out of all religion.
I know you wanted only a paragraph, but sadly, reality isn't simple enough to write Cliff Note versions of Cliff Notes. I had to make it at least this long just to cover the basics.
Added: In a post above, you said we all rely on faith. Yes I do. Faith in my friends, faith that I won't die in a car wreck when I go to the store, etc. Friends have failed me, and though I've been lucky, others haven't made it to the store. So even the faith we rely on is far from infallible. And it in no way resembles the faith of the religious, which is the only thing they have to work with because their god won't show up and say hi. That faith is very different than my faith in the blandness of a McDonalds hamburger.
