By nature, human being does not believe everything right away. First of all we judge the things by our five senses but everything doesn’t meet the measure of touch, taste, hearing, seeing and smelling. More often than not, we restrict ourselves to "Seeing is believing" or try to identify with our conscious mind, which creates an opaque screen of arguments, logic and judgments obscuring the true concept. That is when believing in Reiki comes in.
There are countless energies around us that work beyond our physical senses but we do not believe their existence unless we get acceptable proof. Even if the proof is provided, many remain skeptical about non-physical things. Thus outright rejection of what is not already known to us or understood through physical properties is not uncommon. Such rejection or resistance to ideas only promotes ignorance. There are certain things beyond the scientific proof but that does not make them non-existent.
The universe is full of countless energies. Some of those are known but most of them are not. Reiki energy is one of the known energies but is not believed easily. People want scientific proof of Reiki but they forget the fact that such proof if any, would be presented by others whom they will have to trust and believe their findings. A simpler and definite way is to receive it and feel the results. This can be done either by learning to use Reiki and then applying it or getting few Reiki treatment sessions from a practitioner. Both ways, Reiki will prove not only its existence but also its effects. That will meet the "Seeing is believing" requirement unless the person doesn’t trust even his own senses. Give it a try if you don’t believe in reality of Reiki.