In the Catholic Church, holy water is water blessed by the priest with solemn prayer, to beg God's blessing on those who use it and protection from the powers of darkness.
According to Wikipedia the holy water blessing ritual is:
``Exorcizo te, creatura aquæ, in nomine Dei Patris omnipotentis, et in nomine Jesu Christi, Filii ejus Domini nostri, et in virtute Spiritus Sancti: ut fias aqua exorcizata ad effugandam omnem potestatem inimici, et ipsum inimicum eradicare et explantare valeas cum angelis suis apostaticis, per virtutem ejusdem Domini nostri Jesu Christ: qui venturus est judicare vivos et mortuos et sæculum per ignem.
Deus, qui ad salutem humani generis maxima quæque sacramenta in aquarum substantia condidisti: adesto propitius invocationibus nostris, et elemento huic, multimodis purificationibus præparato, virtutem tuæ benedictionis infunde; ut creatura tua, mysteriis tuis serviens, ad abigendos dæmones morbosque pellendos divinæ gratiæ sumat effectum; ut quidquid in domibus vel in locis fidelium hæc unda resperserit careat omni immunditia, liberetur a noxa. Non illic resideat spiritus pestilens, non aura corrumpens: discedant omnes insidiæ latentis inimici; et si quid est quod aut incolumitati habitantium invidet aut quieti, aspersione hujus aquæ effugiat: ut salubritas, per invocationem sancti tui nominis expetita, ab omnibus sit impugnationibus defensa. Per Dominum, amen.´´
In Supernatural, Holy Water is seen to hurt or repel some demons, although not all.
- In 1.21 Salvation, John uses a rosary to bless water in a tank, and later uses the water to fend off Meg and Tom.
- In 1.22 Devil's Trap, Sam splashes holy water on John when they find him, but there is no reaction even though he is possessed by Azazel.
- In 2.14 Born Under a Bad Sign, Bobby reveals Sam's possession by Meg by getting him to drink beer laced with holy water. Dean also throws a bucket of holy water on Sam before he starts the exorcism ritual.
- In 2.22 All Hell Breaks Loose: Part Two, Bobby gets Ellen to drink a shot of holy water after she reappears following the demonic destruction of the Roadhouse.
- In 3.01 The Magnificent Seven, the hunters use flasks filled with holy water to combat the Seven Deadly Sins. Later, Dean subdues the demon Lust by submerging her in a bathtub of holy water.
- In 3.12 Jus in Bello, Sam steals a rosary from Nancy Fitzgerald. They use this to bless the water in the toilet, which they use to reveal Henriksen's possession.
- In 3.16 No Rest for the Wicked, Bobby blesses the water in a sprinkler system. He uses the sprinklers to create a wall of water that Lilith's soldiers cannot penetrate. It is stated by Ruby that Lilith herself has some immunity to holy water.
- In 4.01 Lazarus Rising, Bobby splashes the resurrected Dean with holy water to prove that he is not possessed.
- In 4.16 On the Head of a Pin, Dean tortures Alastair with holy water.
- In 4.19 Jump the Shark, Dean slips the ghoul disguised as Adam Milligan holy water, but it is not affected.
- In 5.17 99 Problems, the townspeople have a unique means of delivering holy water: a fire engine. They bless the holy water in bulk and spray it at demons with a fire hose.
- In 6.01 Exile on Main St., though he is in theory retired, Dean takes the precaution of keeping a mason jar of holy oil under his bed.
Holy Water: 1.21 Salvation
John says in Latin: "Exorcizo te creature acquae in nomine Deo, patris omnipotentis et in virtute Spiritu Sancti."
Now, acquae sounds more like oque but it only makes sense if it's acquae, lit. 'of water' - however, John's pronunciation sounds a little clearer than Sam's. I don't know if this is due to the different emotional situation in which the two have to pronounce the Latin (Sam's in fact more stressful than John, in both instances) or simply to Jared Padalecki and Jeffrey Dean Morgan's accents.
In English, that sentence translates in: "I exorcise thee, creature of water, in the name of God, father omnipotent and by virtue of the Holy Spirit".
Quick and easy, I do believe that again this is a quick ritual put together by the writers. however, given the previous information found, the fact that it's John (or Sam in Phantom Traveler and Devil's Trap) pronouncing the exorcism and not a priest doesn't make the ritual ineffective.
To be strictly theological, I suspect that it's not an exorcism John should have performed, but a blessing. The exorcism implies that there's a demon in that water, whereas a blessing would have turned the water into Holy Water. So, technically, its a mistake.
The following is the entire Exorcism of Water that was in official use prior to the reforms of Vatican II:
"O water, creature of God, I exorcise you in the name of God the Father almighty, and in the name of Jesus Christ His Son, our Lord, and in the power of the Holy Spirit. I exorcise you so that you may put to flight all the power of the Enemy, and be able to root out and supplant that Enemy with his apostate angels: through the power of our Lord Jesus Christ, Who will come to judge the living and the dead and the world by lire. Amen. Let us pray.
O God, Who for the salvation of mankind has built Thy greatest mysteries on this substance, water, in Thy kindness hear our prayers and pour down the power of Thy blessing + into this element, made ready for many kinds of purification. May this, Thy creature, become an agent of divine grace in the service of Thy mysteries, to drive away evil spirits and dispel sickness, so that every- thing in the homes and other buildings of the faithful that is sprinkled with this water may be rid of all uncleanness and freed from every harm. Let no breath of infection, no disease-bearing air, remain in these places. May the wiles of the lurking Enemy prove of no avail. Let whatever might menace the safety and peace of those who live here be put to flight by the sprinkling of this water, so that the healthfulness, obtained by calling upon Thy holy name, may be made secure against all attack. Through our Lord Jesus Christ, Thy Son, Who lives and reigns with Thee in the unity of the Holy Spirit, God, for ever and ever. R. Amen."**