When I was on a Foreign Study Program in Greece in 2002 we went to Meteora, one of the coolest monasteries ever. We wore long skirts so as not to offend the Greek Orthodox monks. They have a room filled with dead monk skulls. It seemed strange at the time, but maybe not as weird as the tiny petrified body of a little boy that's on display at the Franciscan Monastery of the Holy Land in Washington DC. What's the deal with the dead people (or pieces of dead people!) on display for public veneration? What's the deal with relics?
Assuming there's an afterlife (which we do), then what happens when I die? I leave my body behind, right? I don't need it anymore, right? Well... sort of. But it's still my body even though it's going to rot away. And I will still need it. But don't worry, it will be restored in a miraculous way during the resurrection of the dead. No biggie. Jesus raised Lazarus from the dead and restored him to earthly life even after Martha said, although not in these words, "ummmm... he's been dead for four days. Are you sure you wanna go in there? It's gonna be super stinky." My body is me. It's not just a possession that I own, like my old t-shirt that can be thrown away if I die. Angels may be pure spirit but humans are spirit and body so if I'm going to have a human life after death (instead of one of those creepy ghost lives like in Hades/Sheol) then, at some point, I need my body back! When Jesus was resurrected he let the Apostles poke at him and eat with him so they could make sure he was a living person and not a ghost. A living human being has a body.
So at the end of the world my rotten body, or my cremated body, or my disintegrated body, or my fill-in-the-blank body will be restored and become my 'glorified' body. What happens in the meantime? It's still my body even though it's no longer animated because it's separated from my soul. Sure, in some sense it doesn't matter what happens to it because it'll get patched up later. But it should be respected because it's still part of me.
And--brace yourselves!--my body is a holy object. (Although certainly not as holy as it should be.) When a person is Baptized, he or she becomes a temple of the Holy Spirit, meaning that God comes to live within him or her. Fr. Dale says that when he's celebrating a funeral, he bows to the body while incensing it. Not because he's worshiping the body but because God is still at home in that body. Grace is God's life within us, i.e. the presence of God. A special grace is attached to any object, like a rosary, that has been blessed. When the blessed object is no longer usable it gets either burned or buried so that it won't be defiled. The same thing is done with bodies.
The more we're freed from sin the more 'space' there is for God to live within us (cause God doesn't fraternize with sin). So a holy person, or a holy person's body, will be more strongly steeped in God's grace--hence, the seemingly superstitious practice of venerating relics. The relic is venerated; God, present within the relic, is worshiped. Idol worship? No! Neither the relic nor the person it came from is being worshiped. As usual, God is being worshiped. No surprise there.
What about praying to St. Anthony, for example, in the presence of his relic? Praying in the presence of his relic reminds me that he is still alive and that the spirit world is closer than I realize. And, because his body is still his, I'm in the same room with him and we're praying together! Just like praying in the same room with my friend Jeremy is different from texting to ask him to pray for me. But again, the most important thing about a relic is the presence of God. And if I know this person is a Saint then I know that he is full of grace, because that's one of the conditions of being in Heaven.
A first order relic is an actual piece of the body of a Saint. Lots of churches have sarcophagi to display the bodies of Saints, and sometimes small pieces of tissue are removed to make relics for other churches, like the piece of petrified flesh at the Shrine of St. Anthony in Ellicott City. Sometimes the bones of a Saint are dug up to make relics--there's a tradition of embedding the relic of a Saint in every altar. But just like every other fun thing, sometimes people go overboard. After Catherine of Siena died her body was kept in Rome until her head was smuggled out by some people of Siena. They felt sure that she would rather be kept at home but doubted that they could smuggle the entire body.
A second order relic is something that was owned by the Saint, usually a piece of clothing. The presence of God within the Saint sanctifies his or her clothing and possessions so that they carry a special grace, just like the blessed rosary. Remember that Luke 8:40-48 tells of a woman who is instantly healed after touching the cloak of Jesus. Her faith was strong enough that just touching his garment was enough.
A third order, or higher, relic is a piece of cloth that was touched to a second or third order relic and now retains some extra grace. These minor relics can be bought and sold. In the YouTube video Blessing of Cloth for St. Anthony Relics, Fr. Richard Jacob touches a large cloth to the reliquary of the major relic of St. Anthony, his petrified flesh, in Ellicott City.
But just remember, it's all fun and games until someone desecrates a body!