Clerical Relics

Fun fact: St. Anthony's chapel in Pittsburgh contains
the second largest collection of Catholic relics in the world.

The D&D cleric spell list is, as +Skerples  has pointed out, rooted in Sunday School miracles and Hammer horror movies. This is fine and good if you want to play as Peter Cushing (and let us be fair, that is hardly ever a bad thing), but it perhaps does not get you the most bang for your buck in the field of religious fun-times in your games.

Now, there won't be any new mechanics here, just a method to provide some extra flavor. As such, it should be compatible with whatever cleric variant you are using. (For alternate clerics, see  +Lungfungus' hierophant, +Skerples  generic cleric, or Logan Knight's mystic.) 

Sacraments

First level spells are sacraments. These are the rites and outward signs sacred to your god and the basic building block of the religion. DMs, this is the easy way to change the ground-level flavor of your clerics. Swap out spells, give them some kind of spiritual / symbolic purpose, backronym till your heart is content.  (A single sentence will get you by. Dark Souls away!)

Relics

Mechanically, relics are just scrolls and wands. (But don't tell the wizards that, they'll have a fit. There are three categories of relics, courtesy of the Catholic Church (providing quality gameable material since AD 325.)

First-Class Relics are the physical remains of a saint. Items of direct divine influence or origin would also fall under this category. 

Second-Class Relics are possessions and other objects notable within the saint's life or work. If they belonged to a central figure of the religion, they may classify as first-class.


Third-Class Relics are items touched to a first-class relic, and so have inherited some of the mojo through the commutative property of holiness. Game Usage: treat these as scrolls - a single cast, then it's all used. Note: the wording here might indicate that you could make thousands of third class relics with a single first-class. For game purposes I would say limit it to 1d4, unless you want the big mess.

Based on this, what is to follow is geared towards a pretty standard not-Catholic fantasy church, for the sake of making a baseline example. It's also what I'm more familiar with, so don't be beholden to it on my account - relic classes can easily be swapped out for whatever hierarchy or lack thereof you wish, they all cast the same.

Die-Drop Relic Generator

d4: What Kind of Relic Is it? 

1. First Class (see d12)
2. Second Class (see d10)
3. Third Class (see d8)
4. Fake (roll 2d6)
   

d6+1: What Level Spell Does it Hold?

• Roll on whatever spell table you happen to use for the spell in question.
• Optional: for second-class relics, only count 1-4. For first-class relics, only count 4-6.

d12: First-Class Relics

1. Blood
2. Bone
3. Skin
4. Flesh
5. Hair
6. Head
7. Limb
8. Organ 
9. Tongue
10. Eye
11. Other bodily fluids
12. Entire body

d10: Second-Class Relics

1. Walking stick
2. Book of psalms
3. Prayer beads
4. Sandals
5. Item associated with their trade
6. The means of their execution
7. Original manuscript of a theological treatise
8. Piece of artwork
9. Holy symbol
10. Mark of office

d8: Third-Class Relics

1. Piece of cloth
2. Jar of oil
3. Medallion
4. Holy symbol
5. Prayer card
6. Icon
7. Vial of holy water
8. Incense

d20: Who is This Relic From?

1. Minor, obscure saint
2. Companion to a martyr
3. Popular local saint
4. Warrior saint
5. National patron
6. Martyr
7. Foreign convert
8. Amalgamation of several historical individuals and local folklore
9. Noted scientist or artist
10. Developer of important doctrine
11. Spearheaded reform movement
12. Founder of notable religious order
13. Origin and deeds hidden by time
14. Famous debater of heterodox factions
15. Doctor of the church
16. Figure of legend
17. World-traveling missionary
18. Potent hierarch
19. Prophet
20. Angel or equivalent celestial being

d20 alternate version: Who is This Relic From?

1. Argrabahd, who would preach until he turned purple and passed out.
2. Lox, who could read a man's heart like a book.
3. Shejen, who was almost entirely unmartyable (not for lack of trying, mind).
4. Brother Coal, who ministered to the Russet Country miners for forty years.
5. Altook, who was cooked up in a stew and eaten by yetis.
6. Quen-Liu the Pirate Queen, who bought her canonization alongside amnesty from the government.
7. Elia, who decided that being a virgin martyr wasn't all it was cracked up to be and escaped her captors. Died at 104 after the birth of her fifth great-great-grandchild.
8. Percival Tuckett, who survived thirteen nights of goblin pranks in a failed attempt to convert the warren on Puckawanakee Hill.
9. Anne of Redrosen; gambler, cheat, patroness of the out-of luck. Had her fingers cut off for trying to con the House of Silver Dreaming.
10. Henry Parson, the first priest to come to Tin Jacobstown; chartered the first agreement with the Black Amazons.
11. [Redacted], who has been stripped from the record for posthumously-revealed crimes.
12. The Sable Maid of Orlei, who led the campaign against the Dukes of Hell, and challenged Darvatius himself before the walls of Dis.
13. Orichus Alm, tender of the Angelic Engine and wise teacher of technical theology.
14. Stossisan, who oversaw the building of the  Obsidian Cathedral on the slopes of Mt. Kathualo.
15. Hauteliwika, who lived on nothing but the morning mist and a single acorn a day.
16. Bogab the Ogre, who held the gates of Dunson shut against the invaders with one hand and blessed the soldiers' weapons with the other.
17. Iridalus, one of the rare wizard-converts and author of a masterful catalog of spirits.
18. Grey, who forsook his name and tended to the ghoul-lepers of Janashkut.
19. Kaeltophe, who is said to have fought off ten thousand demons with his enchanted spear, and ridden a gigantic bristleback boar.
20. Navima, who crossed a sea a neophyte, and returned from unknown lands wise beyond all telling.

I am an Idiot and Bought a Fake Relic, What Happens? (2d6)

1. Your knowing and unrepentent blasphemy has called down doom upon you. 
2. Horrific spell backfire! Gain 1d4 mutations.
3. Normal spell backfire. Gain a mutation.
4. You are blocked from intoning the gods for 24 hours
5. You are blocked from intoning the gods for 1d6 days.
6. Nothing happens. Save vs fear, as your faith has failed in time of need.
7. Nothing happens. A spell die is burnt / a spell slot is lost until tomorrow.
8. Nothing happens. There is a sense of relief.
9. Take 1d4 damage from the relic melting in your hands.
10. It works, but the spell's effects (duration, range, damage, etc) are halved.
11. It works, but casts a different spell of 1d4 levels lower.
12. Well what do you know? It was actually real.