I have mentioned before that Shinto is much more open to practitioners of other religions than one might expect from a western perspective, and there was a clear example reported in Jinja Shinpō on January 13th.
On Awaji Island, part of Hyōgo prefecture in central Japan, there is a memorial to the students who were killed in action in World War II after being called up by the government. This was originally built by the local government, but the number of visitors dropped off, and the Hanshin-Awaji Earthquake in 1995 was its death knell in that guise. However, the chief priest of Izanagi Jingū, a major jinja close to the memorial, took the initiative to organise memorial matsuri there, and this year the 25th was held. The group that organises these events is based at Izanagi Jingū.
The memorial matsuri was performed by members of the national association for younger priests, and followed a fairly standard structure. It opened with the singing of the national anthem, which is not standard, but that was followed by purification, the invitation to the kami (in this case, the spirits of the dead students), the presentation of food offerings, and then the prayer or norito (technically called a saishi in this case), which was followed by further prayers offered by the head of the organising group and a representative of the prefectural governor.
After this, a Marine Self Defense Forces band performed, and then miko from Yasukuni Jinja performed kagura. This is the standard place in the ceremony for kagura to be performed, so this is fairly normal.
Next, a Buddhist sutra was read, and a Christian choir sang a hymn. That is not completely standard.
After that, representatives of the attendees offered tamagushi, and made reverence in the normal Shinto way.
The particularly interesting thing about this ceremony is that the timing of the tamagushi offering makes it clear that both the sutra and hymn happened as part of the Shinto ceremony: they were not separate Buddhist and Christian ceremonies performed in the same place, as part of a larger event. The report in Jinja Shinpō certainly does not suggest that this is in any way controversial, and I haven’t become aware of any controversy since.
Of course, in Shinto theology there is no fundamental reason why there should be a controversy. Neither a sutra nor a hymn could be described as inherently disrespectful to the kami, especially as the kami in this case are some of the war dead, and some of them were doubtless Buddhists or Christians in life. Thus, while it is a bit non-standard, it need not be a problem. It is still possible that a problem will blow up in the near future, but I really do not expect it to. Including elements of other religions in a Shinto ritual is not inherently problematic.