We do not discourage non-Christians from joining our boards. In fact, quite the opposite. Go to our home page and have a look at our mission statement.
However, with regard to non-Christians we moderators are tasked with the role of preventing them from undermining the tenets of our faith, ridiculing our beliefs, harassing other members, etc. Sometimes they are very obvious in their purpose, sometimes they are subtle and very good at skirting the rules, and sometimes they are truly inquisitive, looking for answers, or perhaps even interested in Christianity and these are the ones we want to support, share, and witness to with love and respect.
When a seed is planted we don't always know if it takes root but unless we plant that seed, it may never grow.