The reasonable conclusion comes from the vast range of possibilities of what is true, which is exponentially larger than the range of possibilities specifically expressed in the world’s popular religions, even if we were to assume that every human being has their own understanding of what is true. The range of possibilities not conceived of by one of eight-billion human beings is vastly greater, so the chances of one person getting it right is akin to winning the lottery.

If we assume that any two people agree on religious truth, that number of religions becomes less, and the odds it is not one of those becomes even greater.

Note that there are about (not quite) 40,000 denominations of Christianity (and then all the non-denominational churches, some of which are megachurches that stay ND so they are not recognized as an NRM, which law enforcement presumes is a potentially-dangerous cult-or-sect) so we get very specific as to what religious truth is, and we fight wars or litigate over these specifics.

Considering the scope of the universe compared to the scope of life on earth (let alone human life), it’s highly more likely the Milky Way galaxy (including the solar system and everything in it) is incidental to any divine purpose of the cosmos. The difference between the chances that we’re special or important, and the chances mold under a specific Sequoia tree in central California is special or important is infinitesimal.

So even when we only consider theistic possibilities within the universe as we see and understand it, any popular religion that has a non-zero possibility of being true still doesn’t have much more than that.