In episode 16 of Fate/Zero, we see Kayneth go to the church in a wheelchair to plead for a command seal. Risei Kotomine relents, and when transferring one of the command seals onto Kayneth's hand, we see this line:
Take this cup and drink from it. This is my blood, given to you, for the salvation of all. The blood of a contract!
(Translation from Crunchyroll subtitles.) Barring the bit about the contract, this is suspiciously -- to me anyway -- reminescent of the Christian tradition of the Eucharist. However, unlike the real world ritual, there is no actual cup here: Risei says this line while his hand is on Kayneth's.
Is there any in-series meaning or significance to this? (Is it part of the ritual of transferring "blank" command seals unconnected to a specific servant? Notice that when Sola-Ui forcibly takes Kayneth's command marks, nothing similar seems to appear.) Or is this merely symbolism or a reference thrown in for the sake of atmosphere? (Here, perhaps to give a sense of mysticism and foreboding to the Grail, given that blood, on its own, could have negative connotations? Or perhaps simply some sort of "Easter egg"?)
The latter explanation seems more likely to me, given what I've heard about a lack of knowledge of Christianity in Japan, but it could be that I'm missing knowledge about the workings about the Fate series universe.
I have not seen Fate/Zero beyond this. I do have some vague idea of how the Grail works from skimming over various Wiki articles or posts on this site, and from what has so far been covered in the current UBW adaptation.