Thanks for your comment Kanki.

As noted in an earlier comment, the UN Genocide Convention defines it as:

“acts committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group, as such” including the killing of its members, causing serious bodily or mental harm to members of the group, deliberately imposing living conditions that seek to “bring about its physical destruction in whole or in part”, preventing births, or forcibly transferring children out of the group to another group”

Japanese Christians were forbidden to practice their religion on penalty of death — on the basis of their religious beliefs and practices. That pretty well lines up with the definition