I encountered the question this morning in a tweet from Counter Apologist:
Question for Theists:
Is it technically morally wrong to proselytize for something other than the true religion?
— Counter Apologist (@CounterApologis) August 25, 2017
In response, the first point I made is that proselytization is simply the act of attempting to persuade others to hold a particular set of beliefs that you, yourself hold. (I am going to assume, for the sake of simplicity, that one accepts the doctrines in question as true though it is certainly possible to proselytize for beliefs one does not accept.)
With that in mind, we can reword the question with respect to proselytization generally as follows:
Is it technically morally wrong to proselytize for something other than true beliefs?
Now let’s consider an example. Dave and Lee have very different ideas of how the municipal government should address urban gridlock in Los Angeles. Dave believes the government should fund an expansion of mass transit while Lee believes that the government should fund a city-run bicycle loan system. Dave and Lee both present their ideas at a meeting at city hall. Some years later it becomes clear that Dave was correct and Lee was incorrect. Does it thereby follow that Lee behaved in an immoral fashion whereas Dave did not?
Of course not. The lesson is that the truth of one’s beliefs is not connected to the moral assessment of the act of proselytizing for those beliefs.
This brings me to the second point. It is still possible that Lee behaved in an immoral fashion. For example, it could be that the evidence for the superiority of Dave’s mass transit view was so overwhelming that Lee could only retain his commitment to the bicycle loan system by way of an irrational intransigence toward the available evidence.
In that case, Lee could be morally culpable. But note that Lee wouldn’t be morally culpable for proselytizing for false beliefs. Rather, he would be culpable for failing to consider the evidence that contradicted those beliefs.
Christianity and Naturalism
With that in mind, let’s consider for a moment that Christianity is true and naturalism is false. In that case, the naturalist is not culpable for proselytizing on behalf of their (false) naturalistic beliefs. But even so, they could still be culpable for failing to consider the evidence that contradicted those beliefs.