2 Timothy 3:5 having a form of godliness but denying its power. And from such people turn away!
I am not call upon you to judge anyone, but seeing a person and thinking they are a Christian because of the way that they act is not right. We are reminded that God looks on the inside and not on the outside appearance; because appearances can be deceiving!
What makes a person a Christian is their belief in God and all that He can do. If we deny Gods power in any way then we are not giving due reverence to Him.
God does not want us to be duped by great pretenders but by real people of faith.