The job of a sheep is to follow the Shepherd. The job of a Christian is to follow Christ.

You know if a sheep is doing a good job being a sheep if it can be found with the shepherd.

You know if a Christian is doing a good job Following Christ if they can be found with Christ.

The only being that is exactly where the shepherd is is the shepherd himself, but that doesn't make him a sheep, and sheep doing a bad job of it doesn't make them any less sheep.

So too Christ isn't a Christian and the fact that Christians are bad at being Christians doesn't make them not Christians.