Calling someone a Christian, who is not one, is dangerous.
It weakens our testimony;
it keeps us from praying for his greatest need–salvation;
it gives false hope to the one acknowledged;
and it just looks stupid to the watching world.
If a person displays no love for Christ or His people, he is not a Christian.
If he displays no godly characteristics, or Spirit fruit, he is not born again.
By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.
1 John 3:10 ESV
Also: Galatians 5:22