Do I have to become a Buddhist to learn Reiki?
Although the original Reiki system was based on Buddhist Sutra and Tantra, as well as being inspired and developed by Usui who was a devout Buddhist, the way of Reiki does not require formal Buddhist commitments.
So, No, you do not have to be a practicing Buddhist to be attuned to Reiki.
Usui actually studied many religions of his day and was interested in benefiting as many people as possible with his Reiki system.
And wanted Reiki to be available to anyone, no matter the religion they followed or did not follow.
Depending on the Master Reiki teacher you study with, you will see religious influences and styles that their lineage incorporated.
Understand Reiki is available to anyone no matter the religion.
I wish you peace and prosperity