Forget about precious Reddit karma. It is your Christian duty to proselytize.

Proselytizing incorrectly however, does not spread the Christian message and is in fact detrimental to Christianity.     

Be very certain of your facts before you argue with them. Never be disingenuous; they are expecting it and will verify anything you provide as proof.    

Educate yourself so as not to repeat arguments that will not work, for example, do not try any of these [34 Unconvincing Arguments for God](
http://wp.patheos.com.s3.amazonaws.com/blogs/friendlyatheist/files/2007/11/34-Unconv.-Arg.-for-God.pdf). 

Do not be afraid of doing your Christian duty by spreading the message to /r/atheism; but do it effectively. Understand the message you are trying to spread and understand the arguments against it. Even if you are unable to gain converts, you will be educating yourself.