I trained christians, muslims, jewish, buddist and atheists in how to use Reiki they all love it however if seems to work best whenever we detach ourself from anything and cut our EGO out.
What I suggest: before giving Reiki ask in gratitude for the energy to flow (not necessary to a divine being (God) kind a like starting yourself up (anyway are we seperated from any divine?)
Then give Reiki without too much thinking or directing it just let it flow you may ask the energy to flow for the highest good of your healee.
After the session wash your hands and express gratitude that you were allowed to channel life force for a loving purpose.
Of course if you prefer to connect to any God you believe in that is perfectly all right.