Note: This is written by an atheist for other atheists. You can rarely (if at all) convince a believer by an approach like this. Please don’t use this tone when conversing with your believer friends. It will not work.
Any believer I have ever seen so far, without a single exception, have had a moral standard that has been at best equal to the lowest moral standards I keep seeing in non-believers, and at worse a degree of magnitude lower.
That does not prove anything of course. That is a personal observation and can be biased. Although the number of believers and non-believers I have seen is large, still someone can object to the sample being non-representative of the actual population. That can very well be the case.