The reason atheists can (and should) enter into religious discussions and theological disputes, judging religions, is because Jesus, Muhammad, Moses, Krishna, and Buddha do not belong to the faiths to which they are associated.
In the 21st century, these figures belong to global culture. The texts associated with them can be read by anybody in translation and verdicts drawn (and hypocrisies among “followers,” such as Tim Teebow in relation to Jesus, noted). No man is an island. How religious texts are read and interpreted impacts everyone. Therefore, atheists, if they want to be, are properly part of the discussion. It is not ridiculous for them to critique religion and the followers of religion on religion’s own turf and terms.
A Christian no more owns Jesus than a Hindu owns yoga. Below, for example, is a Hindu spiritual practice—yoga—being appropriated by Westerners for, shall we say, other than spiritual purposes.
Are such syncretisms fair game?
Why? Because Hindus don’t own Patanjali (or any other early yoga innovator). No religion gets to escape full public view and dissection on the Internet, nor does it get to own how people apply it or what they notice about its (self-proclaimed) followers. An atheist commenting on Jesus’s followers or a Christian commenting on the Nietzschean consequences of atheism are not out-of-bounds or acting absurdly.
Jesus is not the property of 21st century Christians and Nietzsche is not the property of atheists. Because of our inescapable interconnection with one another in the 21st century, the dialogues invariably cross the boundaries set by earlier, more isolated, historical eras.