Relics are explicitly biblical (2 Kings 13:20-21):

So Elisha died, and they buried him. Now bands of Moabites used to invade the land in the spring of the year. And as a man was being buried, lo, a marauding band was seen and the man was cast into the grave of Elisha; and as soon as the man touched the bones of Elisha, he revived, and stood on his feet.

Veneration is essentially different from worship or adoration (reserved for God alone); it is a high honor given to something or someone because of the grace revealed or demonstrated in them from God. The relic (and the saint from whom it is derived) reflects the greatness of God just as a masterpiece of art or music reflects the greatness of the artist or composer.

Therefore, in venerating it, God is being honored. The saint is being venerated only insofar as he or she is reflecting God’s grace and holiness. If such an item is worshiped, the person doing it is not following Catholic teaching, which fully agrees with Protestantism with regard to the evil of idolatry, or putting something besides God in the unique place of God.

In the passage above, matter clearly imparted the miraculous and grace from God. That is all that is needed for Catholics to reasonably and scripturally hold such items in the highest regard and honor (veneration).

There are other explicit biblical examples of relics:

• 2 Kings 2:14: Then he took the mantle of Elijah that had fallen from him, and struck the water, saying, ‘Where is the Lord, the God of Elijah?’ And when he had struck the water, the water was parted to the one side and to the other; and Elisha went over.”
• Acts 5:15-16: “They even carried out the sick into the streets, and laid them on beds and pallets, that as Peter came by at least his shadow might fall on some of them. The people also gathered from the towns around Jerusalem, bringing the sick and those afflicted with unclean spirits, and they were all healed.”
• Acts 19:11-12: “And God did extraordinary miracles by the hands of Paul, so that handkerchiefs or aprons were carried away from his body to the sick, and diseases left them and the evil spirits came out of them.” (cf. Matthew 9:20-22)

Elisha’s bones were a “first-class” relic — a relic from the person himself. These passages, on the other hand, offer examples of “second-class” relics —  items that have power because they were connected with a holy person (Elijah’s mantle and even St. Peter’s shadow) — and third-class relics, or something that has merely touched a holy person or first-class relic (handkerchiefs that had touched St. Paul).

In the Pentateuch, we have a remarkable foreshadowing of relics and specifically of receiving holiness as a result of touching sacred objects:

• Exodus 29:37: Seven days you shall make atonement for the altar, and consecrate it, and the altar shall be most holy; whatever touches the altar shall become holy.
• Exodus 30:25-29:  And you shall make of these a sacred anointing oil blended as by the perfumer; a holy anointing oil it shall be. And you shall anoint with it the tent of meeting and the ark of the testimony, and the table and all its utensils, and the lampstand and its utensils, and the altar of incense, and the altar of burnt offering with all its utensils and the laver and its base; you shall consecrate them, that they may be most holy; whatever touches them will become holy.

Note here that first there is a “holy anointing oil” which is applied to the ark, tabernacle, and related sacred religious items “that they may be most holy.” They in turn impart holiness to all who touch them. If this is not sacramentalism and the underlying principle of relics, nothing is.

God said to Moses about the body of a lamb offered at the temple: “Whatever touches its flesh shall be holy ...” (Leviticus 6:27). So now we again have a dead thing (like Elisha’s bones) imparting holiness. How is that any different from Catholic relics? Likewise, the same was said even of the cereal offering (Leviticus 6:14-18).

The ark of the covenant had relics in it, too: “a golden urn holding the manna, and Aaron’s rod that budded, and the tables of the covenant [the two tablets of the Ten Commandments]” (Hebrews 9:4; cf. Exodus 16:33-34; Numbers 17:10; 1 Kings 8:9; 2 Chronicles 5:10). The manna (a type of food produced by God to feed the wandering post-Exodus Jews) was biological, so presumably it had to be supernaturally preserved if it lasted very long. The parallel to relics and the practices and principles connected to them is very clear.

Protestant critics of relics will ask where we should draw the line between a proper use of relics and a corrupt, idolatrous one. If they become idols in place of God or are used for financial gain, or are thought to be magic charms (superstition), that’s wrong, and the line has been crossed. The understanding of them has to be sacramental and incarnational, and grounded in a proper biblical understanding of the veneration of saints.

Superstition and idolatry are — like lust or pride or greed — erroneous and wicked attitudes that reside in someone’s heart. We don’t usually know if this is what they are thinking simply by observing outward actions. Two people could be bowing before a relic. One is in fact (if we knew their inner attitude) viewing it as a charm or an idol, and is gravely sinning. The other is venerating it, which is perfectly biblical and Catholic. So the lines are difficult to determine, based on these inherently subjective factors. It’s not a simple matter.

The word relic (like Trinity) is not in the Bible, but the Bible does teach the nature and concept of the doctrine.