Kempis is a very storied and well-respected theologian from right before the Reformation. He's looked upon fondly by the Anglicans, Methodists, and Jesuits alike. He's about as Christian as they come, and the fruit of his belief is abundant.
Thomas Kempis is very much Christian. There are a variety of Christian authors in this vein. Modern American Evangelicalism doesn't comprise the entirety of religious thinking.
You cannot say who is or isn't Christian any more than I can. Just because you reference those YOU see as an authority didn't make you correct. It simply moves the problem one step to the left. Those people also can't say who is or isn't Christian.
There is no metric by which you can measure who is a Christian. At best you can say that some people don't act as you would expect Christians to act. But that's just your option, and says more about your beliefs than them.