Why are there so many lions around Venice, and did they really steal the corpse of St. Mark?
There are lions all over the city of Venice because that it the city's symbol.
It is the city's symbol because this is the city of San Marco—Mark the Evangelist—and the lion was St. Mark's symbol.
The theft of St. Mark
In 828 a group of Venetian merchants were visiting Alexandria.
In Alexandria rested the bones of the famous and oh-so-holy St. Mark the Evangelist.
So, of course, the merchants decided to steal him.
This was era when acquiring bits of bona fide saints was de rigueur for relic hunters.
Most folks settled for the finger bone of St. Elias, or the shriveled skull of St. Catherine, or a fragment of the ulna of St. Luke.
The Venetians figured: what would be better to carry home in patriotic triumph than a bit of St. Mark?
How about: Carrying home all of St. Mark.
According to legend, the merchants exhumed the Evangelist's remains and smuggled them out of town in a barrel of pickled pig parts,cleverly banking on the fact that Muslim proscription against even touching pork would help them slip through inspections.
The merchants returned home with their prize, and the city promptly set about building an appropriately extravagant church to house the holy remains.
The result: St. Mark's Basilica, and the adoption of Mark's iconic winged lion as the new symbol of the city. (Fun aside: Learn why the Evangelists—Matthew, Mark, Luke, and John—are symbolized by a man, a lion, an ox, and an eagle, respectively.)
[Incidentally, Venice did have a patron saint before the, er "aggresively adopted" St. Mark. It was St. Theodore—and you can still see him, along with his dragon, perched atop one of the two ancient columns thrusting toward the sky beside the Doge's Palace where Piazzetta San Marco meets the end of the Grand Canal. Atop the second column? A winged lion, of course.]
The great saint steal-a-thon
The audacity of the Venetians kick started something of a grisly competition between Italy's hyper-competitive maritime capitals to see who could steal the best saint then build a cathedral around his bones.
In 1087 Bari, in Apulia, countered by nicking the 4th century Turkish bishop St. Nicola di Myra, a.k.a. St. Nicholas,a.k.a. Santa Claus.
What happened to the other Evangelists?
In the interests of rounding off the subject:
Most of St. Luke is now just down the road in Padova—except his head, which is in Prague.
(Fun aside: According to Orthodox tradition, Luke himself was a relic hunter! On a pilgrimage to Samaria, he supposedly brought the arm of St. John the Baptist back to Antioch with him.)
St. John is... tricky.
As with most early saints, there is much scholarly debate on just who St. John was—there were a lot of "Johns," some of whom may or may not have been the same guy.
However, the traditional story goes that John was the only Apostle not to be martyred, and that he lived to a ripe old age preaching to his own followers in Ephesus, where he died and which retains his tomb... but not his bones.
When his (said-to-be-miraculous) tomb under the (now-destroyed) basilica named for him was opened in the 4th century, it was found to be empty. Was it a victim of the earliest of church relic hunters?
True believers have their own story: that John had been informed by Jesus himself in a vision of the day when he would die, and that he entered a cave in his little hillside church whereupon vanished in a flash of light, assumed bodily into Heaven.
Sound far-fetched? Perhaps. But the fact remains that, even with tens of thousands of saintly bones nestled in gilded chruch reliquaries around the world, St. John remains the only major saint to have no church anywhere on earth claiming a holy relic in his name.