The religious group hasn’t been named. It appears that Abe may not have been a member of the group, but the grudge against Abe is because Abe funneled government money to the group. The religious group is apparently involved in spying for the Japanese government, hence the funding.
The assassination of Shinzo Abe has drawn attention to a fringe political party that had criticized him ahead of Sunday’s upper house election for alleged connections to religious groups.
Social-media users have highlighted comments by Akihiko Kurokawa, secretary-general of the NHK Party, alleging during a campaign debate last month that Abe was to blame for obscure funding of religious groups in Japan allegedly used as a front for “foreign spy activities.”
Not sure, but I believe I read somewhere that it was Soka Gakkai. I also seem to recall that they were ‘excommunicated’ by the traditionalist Nichiren Shoshu.