See:
the cult of relics
One of the more curious religious relics floating around during the Middle Ages along with the more well known Shroud of Turin and pieces of the true cross.
The concept behind The Holy Foreskin hinges on the fact that, though he himself spoke about the unimportance of physical circumcision, being a Jew himself, Jesus was circumcised. Now, most of the body of The Lord would be unavailable for relics because it was assumed into heaven whole after the resurrection, but the foreskin, having been cut off before his crucifixion, would have been left on earth, and thus, a relic.
There is a saint who claimed to have had a dream in which he was married to Christ with this relic as the wedding ring.
Yes, sometimes truth IS stranger than fiction.