An appeal to antiquity is the opposite of an appeal to novelty. Appeals to antiquity assume that older ideas are better, that the fact that an idea has been around for a while implies that it is true. This, of course, is not the case; old ideas can be bad ideas, and new ideas can be good ideas. We therefore can’t learn anything about the truth of an idea just by considering how old it is.
(1) Religion dates back many thousands of years (whereas atheism is a relatively recent development).
(2) Some form of religion is true.
This argument is an appeal to antiquity because the only evidence that it offers in favour of religion is its age. There are many old ideas, of course, that are known to be false: e.g. that the Earth is flat, or that it is the still centre of the solar system. It therefore could be the case that the premise of this argument is true (that religion is older than atheism) but that its conclusion is nevertheless false (that no religion is true). We need a lot more evidence about religion (or any other theory) than how old it is before we can be justified in accepting it as true. Appeals to antiquity are therefore fallacious.