The search for the Holy Grail is a ‘teaser’ used by occult, quasi-masonic groups to divert people away from Christianity - gives rise to notions of being a knight (no one ever wants to be a page or a serf, funnily enough!)
The cup is not important, it’s the wine it contains.
The Shroud is authentic, depending on who you talk to. Latest scientific tests suggest it is authentic. The later (radio carbon) date previously given was based on a fragment of cloth taken from medieval repairs to the artefact. The original shroud cloth dates from the time of Christ.
Then there’s the odd thing that it’s a photo-negative, photography only being invented recently. The fellah who developed the first photo of it nearly dropped the glass negative when he first saw it!
Be aware that a lot of half-educated people like to speculate about the Holy Grail, Turin Shroud, Kinghts Templar, etc.
I read somewhere that if someone brings the Knights Templar up in a conversation, it’s a good indication they’re loony.