"True" Christianity

Since when is it "ok" for Christians to determine who is or isn't a "true" Christian? I was under the impression that that determination was left to God. The requirement to be a Christian (to believe in one's heart that Jesus Christ is the son of God) is one that would require you to really know the person to even begin to determine. And even then, does it really matter?

With the exception of needing to know that people in spiritual leadership above you are really Christians -- it's not our mandate to ferret out the "imposters". It's our mandate to love others as we love ourselves. That's it. Finito. LOVE.