"It does not do to dwell on dreams and forget to live."
--Albus Dumbledore, Harry Potter and the Philosopher's Stone
1. It's a scam that hurts people
People are promised bliss and riches, after death, and the price is freedom.
2. Its values are backwards
Suffering is bliss, abuse is love, truth comes from imagination and wishes, dreams are real, reality/nature is corrupt, death is gain while life is meaningless.
3. It takes away one's moral autonomy
Followers are told judgement is a sin. It turns morality into an imaginary, arbitrary, subjective standard instead of a system based on the facts of causality
4. It causes humanity to hate and distrust one another
As stated in number two, nature is corrupt because Man is corrupt. Man cannot be trusted and needs to be watched over by a Transcendent Authority with a Transcendent Standard to mete out punishment. Individuals are given imaginary differences based on bloodlines/"nations," and are encouraged to prize these over Humanity itself. Some bloodlines/"nations" are told that the Authority has judged these "special." Through this the Authority and its scripture designates these different labels as "enemies.
That about covers Christianity. Please note that the terms "Government," "State," and "Democracy" can replace the terms "Authority" and "Scripture" in most cases.