Though I left the church long ago, I grew up as a very serious Christian so I know just how important this question can be.
You may have heard that reiki is a universal energy and its use is non-religious and non-denominational. While this is basically true of modern Westernized reiki practices which are primarily focused on physical healing and relaxation, the deeper truth is that reiki originated as a way to reach Enlightenment in the Buddhist sense. So it is still inherently spiritual. Furthermore, the higher levels of all reiki practices still use symbols borrowed from Buddhist texts. And although as a reiki master I do consider its energy to be from the universal source, I realize this concept itself can be problematic in the basic Christian worldview.
What this means for you is that:
If you are a pretty liberal Christian who basically believes in God and thinks the compassionate teachings of Jesus are something we all should be following, but you don't necessarily attend church regularly, and maybe you do yoga, listen to Enya, and have a few crystals lying around the house, then I think you would probably be fine to receive reiki treatments. And especially if you feel like God is a universal source that might be accessible through other ways and faiths, you could perhaps even become a first-level practitioner. The important thing is to check in with your personal connection to God and follow the answers from your heart...
On the other hand, if you are, or have recently become, a fairly strict Christian and regular churchgoer who tends to interpret the scriptures literally, I think the Buddhist origins of reiki are probably going to cause you a lot of internal conflict sooner or later and I wouldn't recommend that you receive or practice reiki at all.
Either way, while you really shouldn't worry if a well-meaning acquaintance tries sending you reiki without you asking for it, if you have intentionally received reiki treatments or practitioner attunements and now have regrets, please talk with your pastor about it or feel free to contact me directly for some free advice in line with biblical teachings to help you out of such a situation. Your spiritual peace of mind is so invaluable to your overall wellbeing--