Acts 19 The Disciples of John

What does it mean to be a Christian?  Is everyone a Christian who calls himself one?  If they say all the right words, does that make them a Christian?  In this passage of Scripture Paul encounters 12 men who are called disciples and he references when they believed.  On might assume from those two words that these 12 were already Christians when they met Paul.  But as the story unfolds, we discover that they are not.