The one who says he abides in Him ought himself to walk in the same manner as He walked. (1 John 2:6 NASB)
According to a poll taken in May 2011, 92% of Americans claim to believe in God, and 78% of Americans claim to be Christian.
Saying you are something does not make you that thing!
I can say I am a plumber, but I am not a plumber just by saying it. I can listen to talk of plumbing, but that does not make me a plumber. I can own any and every item pertaining to plumbing, I can sing about plumbing. I can dress like a plumber(wouldn't want to), but none of that makes me a plumber.
And even if I trained and learned all about plumbing, if I don't continue to use those skills and actually perform the skills, I am not a plumber.
In the same way, to be a Christian, you must abide (live, practice, perform, walk, talk, sing, etc.) in the Christian life, just as Christ did while on this earth, as much as is humanly possible.
Live in Christ. Live in His teachings.