This is a good question. I am sure that different people will have different points of view, but for something to be Christian a few key elements should be present. A good way to get at the heart of this question is to ask what elements are central to Christianity.
First, for a play to be Christian, it should have something to do with the Christian message. The Christian message at its core is about the death and resurrection of Christ. So, this dynamic of death and resurrection should be there.
Second, if the play wants to be explicitly Christian, then there should be mention of Jesus Christ. This point, coupled with the message of death and resurrection, will make the play unambiguously Christian.
Finally, any reference to the New Testament and the many stories there will also be a good way to make something Christian. For example, you can rework many of the stories of Jesus from the gospels - Jesus turning water into wine, raising the dead, cleansing lepers, etc.