A while back, a Protestant friend of mine wanted to understand what relics were, and how it was different than magic or idolatry. I think the Catholic belief in relics can be traced to Scripture and to the earliest days of the Church, so I thought I'd do my best to lay that out today.
What Does the Church Teach?
To begin with, what does the Church teach about relics? So far I know, the Catechism has only one entry that mentions them, and it says: 1674 Besides sacramental liturgy and sacramentals, catechesis must take into account the forms of piety and popular devotions among the faithful. The religious sense of the Christian people has always found expression in various forms of piety surrounding the Church's sacramental life, such as the veneration of relics, visits to sanctuaries, pilgrimages, processions, the stations of the cross, religious dances, the rosary, medals, etc.
In other words, relics are one way of drawing us closer to God. We're not saying that the veneration of relics is indispensable to salvation, and we're not saying that you should (or may) worship relics. “Veneration,” called dulia in the Latin, is a sort of religious honor, the way we're humbled by manifestations of holiness.
Think about it this way. There are folks in this world who you'd be humbled to be next to -- the folks you consider your personal heroes. That's not a bad thing -- in fact, it's a sign of humility. Of course, it can be a bad thing, if your heroes are bad role models. But with the Saints, the Church points us towards heroes of the faith, something of a Hall of Fame of those who have run the race before us. This is all very Scriptural: Hebrews 11 does the example same thing with the heroes of the Old Testament, declaring that “the world was not worthy of them” (Heb. 11:38). That is, Hebrews 11 praises both God working through the Saints, and the Old Testament Saints themselves.
So that sense of wonderment -- where we see the work of God lived out in a person's life, and we honor and respect that -- that's probably the best way I can describe “veneration.” I should also point out that Catholics venerate the Holy Bible. We don't think it's God, obviously, but we recognize that God is at work in the Bible in a unique way, distinct even from how He's at work in the lives of the Saints. I suspect that most Protestants venerate saints and the Bible in their own private way, they just don't call it that.
So with relics, we've got the same thing. These are the bones, clothes, etc., of the great Saints who have gone to God before us. But beyond simply being an object of religious honor, to remind us of God's work in the world, and those who have gone before us in Faith, we believe that miracles can actually happen through the use of relics. It's this last belief that gets labeled as idolatry or superstition, so let's see what the Bible has to say.
What Does Scripture Say About Relics?
(1) The starting point for an examination of Scripture on this point should be Acts 19:11-12: God did extraordinary miracles through Paul, so that even handkerchiefs and aprons that had touched him were taken to the sick, and their illnesses were cured and the evil spirits left them.
That's what Catholics believe about relics, nothing more, nothing less. Look at the elements:
- The objects in question are “handkerchiefs and aprons that had touched him,” what Catholics would call Paul's relics.
- These relics are producing miraculous healings, without Paul doing anything.
- These healings are still a way that God performed “extraordinary miracles through Paul.”
- These are actual miracles, not simply some placebo effect.
So the first thing we can say is that what Catholics believe about relics seems very much to be how Scripture describes relics. There are people healed because they got to touch a relic of St. Paul's. Butwhy does God do this? I think that there's a hint in the fact that these are counted as “extraordinary miracles through Paul.” It's a sign of God's favor on a particular Saint. One of the ways we determine if a given person is in Heaven is if God performs miracles when we pray to that person. If He does, it seems to confirm that His favor is upon them, just as the Resurrection showed that the Father's favor was upon Jesus Christ. Finally, though, the fact that Paul doesn't actually do the miracle -- God does -- shows that while the Saints are honored and blessed by God, He's still the one in the driver's seat. The Saints lead us to God, and always refused any attempt to be made into gods themselves (see Acts 14:11-15).
(2) But these miraculous relics weren't limited to Paul. In Acts 5:12-16, we hear The apostles performed many signs and wonders among the people. And all the believers used to meet together in Solomon’s Colonnade. No one else dared join them, even though they were highly regarded by the people. Nevertheless, more and more men and women believed in the Lord and were added to their number.
As a result, people brought the sick into the streets and laid them on beds and mats so that at least Peter’s shadow might fall on some of them as he passed by. Crowds gathered also from the towns around Jerusalem, bringing their sick and those tormented by impure spirits, and all of them were healed.
Again, we see the same thing: a relic of the Saints (here, the mere shadow of Peter) brings about innumerable blessings, miracles, and healings.And the reason is for the same reason that the Apostles were performing “signs and wonders among the people.” It showed that these men come from God, and that their message is true.
(3) We see this in the life of Christ Himself. Mark 5:25-34: And a woman was there who had been subject to bleeding for twelve years. She had suffered a great deal under the care of many doctors and had spent all she had, yet instead of getting better she grew worse. When she heard about Jesus, she came up behind him in the crowd and touched his cloak, because she thought, “If I just touch his clothes, I will be healed.” Immediately her bleeding stopped and she felt in her body that she was freed from her suffering. At once Jesus realized that power had gone out from him. He turned around in the crowd and asked, “Who touched my clothes?”
“You see the people crowding against you,” his disciples answered, “and yet you can ask, ‘Who touched me?’” But Jesus kept looking around to see who had done it. Then the woman, knowing what had happened to her, came and fell at his feet and, trembling with fear, told him the whole truth. He said to her, “Daughter, your faith has healed you. Go in peace and be freed from your suffering.”
This is a very different miracle from the ones we're used to. Here, simply touching the clothing of Christ heals this woman, before He's even aware that she's there. Mark actually makes a point of including the fact that He felt the power go out from Him, but still asked who it was who had touched Him. And Jesus ascribes the healing to the woman's faith, although it's clear that His own poweris the operative power of healing. She's got the faith to believe that simply to touch something worn by Christ is sufficient to be healed. Some Protestants consider that superstition, but our Lord apparently does not.
Elisha died and was buried. Now Moabite raiders used to enter the country every spring. Once while some Israelites were burying a man, suddenly they saw a band of raiders; so they threw the man’s body into Elisha’s tomb. When the body touched Elisha’s bones, the man came to life and stood up on his feet.
This passage is important, because it shows that the Saint isn't actively performing these, in the way that other miracles are performed. Even after Elisha's dead, God works through him. That's the reason relics work, at the end of the day: because God wills to use all sorts of things to bring about healing and salvation. But He doesn't choose randomly - in every case, it's either been His Son or the Saints who He works through.
(5) Finally, we know that the early Church used relics, as well. That is, just as Elisha's bones show that healing relics pre-date the New Testament, the early Church shows that they continue on afterthe New Testament. Let's take just a couple example. First, through Eusebius (c. 263-339 A.D.) and Caius (early 200s), we know that the early Church kept the “trophies” of St. Peter and St. Paul in the Vatican.
St. Augustine, beloved by Catholics and Protestants alike, said that in Book XXII, Chapter 8 of City of God that:
For even now miracles are wrought in the name of Christ, whether by His sacraments or by the prayers or relics of His saints; but they are not so brilliant and conspicuous as to cause them to be published with such glory as accompanied the former miracles.
That's pretty clear. Likewise, St. Ambrose of Milan, writing in the latter half of the fourth century, wrote:
For after I had dedicated the basilica, many, as it were, with one mouth began to address me, and said: Consecrate this as you did the Roman basilica. And I answered: "Certainly I will if I find any relics of martyrs." And at once a kind of prophetic ardour seemed to enter my heart.
Why should I use many words? God favoured us, for even the clergy were afraid who were bidden to clear away the earth from the spot before the chancel screen of SS. Felix and Nabor. I found the fitting signs, and on bringing in some on whom hands were to be laid, the power of the holy martyrs became so manifest, that even whilst I was still silent, one was seized and thrown prostrate at the holy burial-place.
The first paragraph refers to a Catholic practice - in consecrating a Church, we embed the relics of a Saint. As you can see, this custom is not new. A little later, in the same letter, Ambrose writes:
On the following day we translated the relics to the basilica called Ambrosian. During the translation a blind man was healed [...] They [the Arians] deny that the blind man received sight, but he denies not that he is healed. He says: I who could not see now see. He says: I ceased to be blind, and proves it by the fact.
So relics were widely used, venerated (there are folks falling prostrate!), and are bringing about miraculous cures. What we see is that from the Old Testament, down through the New, through the times of the early Church, all the way to the present, the same central things have been believed about relics. Catholics simply conform with this Scriptural belief. It's not idolatry or superstition. It's good old-fashioned Bible religion.
Finally, note that Catholic Answers has a great list of early Church writings about post-Apostolic miracles, some of which involve relics, here; and a helpful entry on relics here. 
None of these things, therefore, is the explanation of its own existence or the source of its own existence. In other words, their existence is contingent upon something else. Each thing possesses existence, and can pass on existence; but it did not originate its existence. It is essentially a receiver of existence. Now it is impossible to conceive of a universe consisting exclusively of contingent beings, that is, of beings which are only receivers of existence and not originators. The reader who is taking his role as explorer seriously might very well stop reading at this point and let his mind make for itself the effort to conceive a condition in which nothing should exist save receivers of existence.
