The favourite Christmas song 'Stille nacht' or 'Silent night' was composed at the request of a Catholic priest. c.1818 for performance at Midnight Mass in an Austrian Catholic church.
Father Mohr ws apparently a good amateur guitarist and he accompanied the song himself.

http://en.wikipedia.org/wiki/Joseph_Mohr

Yes, guitars are out. Why is this even a question?

The human voice is the most perfect instrument, and why would you want to replace perfection with something less than perfect?

Is there any decree or any quote somewhere?

No it is actually not obvious that guitars are profane. The guitar is simply a plucked string instrument. Would one say that the harp is obviously profane? Probably not because, for lack of any other reason, a tradition of painting angels playing the harp - well, there are plenty of paintings of angels playing the lute and the guitar as well.

I cannot cite any particular book, but I am willing to bet, based on my knowledge of Western musical practices, that the guitar was used in some liturgy some where before Vatican II. I'm almost certain that the lute was used, and also the harpsichord. The guitar is really just in the same family, so I see no reason why it is more "profane".

Now how useful it can be in the liturgy is a different question. In fact, I think that the guitar would provide unintrusive accompaniment to Gregorian chant. Its one short-coming is it's inability to sustain tones for an indefinite period of time like the organ. This alone might disqualify it's potential use in the liturgy, but certainly not some uninformed and unsupported assertion of its inherent "profaness".

Electric guitars might seem like a different matter, but as long as the tone is subdued and imitates that of the acoustic guitar, I don't see the problem. Most non-musicians have no qualms about making organists play on electric mockeries of organs. And even a real organ is about as mechanical as you can get. It's an amazing work of artifice, it's quite humbling to think about what sort of genius it must have taken to even conceive of the idea.


Re: the human voice is perfect, etc.

Read the Psalms. Musical instruments can be used to praise God. Attempts to suppress instrumental music are all failures. Instrumental music is tied in with what it means to be human. Use of instruments is not meant to replace the human voice, but to accompany and compliment.

I recommend perusing the Catholic Encyclopedia entry on musical instruments in the liturgy. Especially the final paragraph. I would quote it but I'm on my phone and that's a hassle.

Instrumentation is a disciplinary matter; the church has altered her views several times on which instruments may be included, according to their use and reputation in secular society. The voice is always preferred, followed by the organ.

Aside, and you may deem this a mere opinion and a 'rigorist' one at that, but it seems clear that from as early as the 15th century there was a great deal of aestheticist liturgical abuse occurring, with overly complex instrumentation. I don't think it's correct to cite willy-nilly from that time period on to justify as traditional whatever instrumentation you may please.

Since this is a disciplinary question there is no essential or a priori reason that guitars should be excluded, but there are important cultural reasons having to do with its informality, whether folksy or punk. SSPX priests have told me that stringed instruments should not be played in church, and I've taken their word for it. I know I would be shocked to hear one during mass, however humbly and classically played.