Should we judge a group by a small number of the members?

There are many times that people will say in comments on the other blogs I follow that this or that person wasn't a true christian. I think that we can base a lot of what we think of the whole group by this small number. I'm not talking about the ones that use the group they are in to hide their immoral activities but on the ones saying who is and isn't true to the group. What they show me is that to them, they must do everything they can to paint a picture of what christianity should be not what it is. Most of the christians I know only apply their beliefs when it is to their advantage or their groups advantage. One of the ways I point this out is to ask them if other forms of christianity are true christians. A few will say yes but most say that their form of christianity is the only true form of it. I then bring up if that is true then the numbers they claim to support their view that the USA is a christian nation are wrong. If you remove all the people they claim aren't true christians you end up with less then 23% of americans are actually christian. By their logic they should be saying we are a nation based on something other then christian beliefs. The 23% is because that is about what the catholic numbers are. When you break it down by denomination you find most are under 10%.