Christianity is not arbitrated by human beings and human organizations. Christianity is a personal relationship with God. Christians will of course, love the Lord, they will obey Jesus Christ. But, simply attending any church, no matter how biblically correct that church may be no more makes any of us a Christian than going out and sitting in a chicken coop makes us a chicken. To return to the force of your question.
Christians have a duty, according to the Bible, to warn others of false teachers and teachings.
Christians also have a duty to love others, and look to the heart of their fellow human beings. Of course, some atheists have good attitudes. Some followers of Islam and Buddha have wonderful attitudes - but their beliefs are not in accord with the Bible. Perhaps speaking the truth in love will cause some to question the false teaching they have simply accepted for many years. Perhaps our warm and considerate approach to them will also help - as opposed to an antagonistic, haughty, self righteous approach.