Just because someone claims to be a Christian doesn't make it so.
This statement while correct, doesn't bring to light the full definition of the problem that many seem to suffer.
use the following;
{Religion} = X
Just because someone claims to be a {X} doesn't make it so.
and thus we have the problem of the world in relation to religious argument in one single statement. Remember the religious moniker that one stands behind during extreme actions is usually there due to the need for self justification or as something to gather support for a completely different personal set of reasons using it to gather others from the periphery to support a lone extremist idealism.
But every system of belief religious, political, or whatever, lends itself to extremity which must be avoided. This isn't new and has been problematic since the beginning of history.
You cannot blame a group for the actions taken under it's name unless there is direct support. Just because a Christian doesn't proactively root out those who would bomb an abortion clinic doesn't mean they have any responsibility. That logic would support the obviously false statement that VT students condoned what Cho did. Obviously this is untrue.