This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
禛 combines 示 (altar, spirit) with 真 (truth, reality), likely as a phono-semantic compound where 真 contributes the sound and possibly a meaning related to sincerity or genuineness in a spiritual context. The exact historical development is uncertain.
The left side 示 suggests something spiritual or ritual, and the right side 真 suggests truth or reality. Together they might hint at sincere spiritual practice, but the exact meaning is unclear.
For シン, imagine a sincere (真) person making an offering at an altar (示): the sincerity of the scene cues シン.