Oh, and there is no such thing as “secular instrument.” That is so ridiculous I can’t put it into words.
It’s not ridiculous at all, and the concept was used by popes, the Second Vatican Council, and the Curia. You need to ponder the meaning, perhaps. I think you might not quite understand what it means. I’ll be quite interested to see if the poster who sends this to the bishop will get a response, and what that response will be. Especially since so many people are so shamefully unwilling to submit themselves to the decrees of the Vatican. This isn’t a matter of opinion; it’s disciplinary. We don’t get to choose. We were told.
Now I wish to address some things that don’t seem to quite be sinking into the pro-guitar crowd’s head: “secular” and “profane” do not mean Satanic any more than “Vulgar” means full of swear words and crude jokes. “Secular” and “profane” refer to things having to do with the world. They are simply terms used to refer to “non-sacred” things, not sacrilegious things. So to say that there are “secular” instruments is of course perfectly sensible. Most of them are secular. That is, they were developed for use in the musical traditions outside of the church. Actually, this is true of most of them, but some are more closely tied to church music. The organ would be the pinnacle of such an example, but the trombone is also another good example of an instrument with a rich liturgical tradition.
“Secular” isn’t just an opinion. It’s everything that is outside of the church’s musical tradition, since everything that isn’t part of the tradition of sacred music is, by definition, secular. That’s not a condemnation. I love secular music. It’s perfectly healthy and normal to listen to it. Our pope loves Mozart, and most of his music is secular. There’s nothing bad about secular anything, as long as it is not sacrilegious. And while the secular music of the people changes from culture to culture, the sacred music proper to the Roman Catholic church does not. Therefore, if we look at the banjo in Europe (where it is not used), it does not become a sacred instrument just because it isn’t part of Europes secular music tradition.
The problem is that many of you are not willing to go back and look to the sacred musical traditions of the Catholic church. We need to develop a better understanding of history. First and foremost, no translation of the Bible renders the names of the instruments well. The instruments that they used back in the days of early Israel are not the same instruments that we use today. Using the word “harp” conjures up an image of an instrument that simply did not exist then. “Trumpet,” “tambourine,” and so forth are troubled by the same problem. Beyond that, these are instruments that David mentions as being used in what we can only understand and private devotion. If you’d like to listen to guitar music about God outside of mass, you are free to do so; it is healthy and good, so long as the lyrics are orthodox. But this is the equivalent of what David is talking about. There is no mention of instruments being used int he Temple, except for the shofar, which is a horn trumpet–a wind instrument closer to the organ (which has trumpets as one of its stops) than a guitar.
The music of the Temple was vocal. They chanted. The early church chanted. St. Paul refers to the early church singing, but never playing instruments. That is chant (not yet Gregorian, but nevertheless chant). They chanted and chanted and chanted till something magical happened in Notre Dame in Paris during the middle ages: composers began to put multiple lines of chant together and created polyphony. It was the first recorded instance of harmony of any type–invented by the sacred musicians of the Catholic Church. Throughout the late Middle Ages and Renaissance polyphony was developed, with the use of the organ to accompany the voices. Other instruments were also used: we have record of various brass instruments being used to double the parts of the choir or to play the parts of a second choir. The violin family (violin, viola, and 'cello) were later brought in along with other similar instruments to be used in similar manner to the brass: to double the choir, or to play as a second choir by the end of the Renaissance.
Now here’s the break-- during the Baroque, there was a sudden change in musical aesthetic. Some composers decided to write sacred music in this new aesthetic, although these were mostly Protestant. Meanwhile, many composers continued to write in the “stile antico”–the “old style.” They mimicked the works of the earlier Catholic composers. Many of these composers wrote their secular music in the new style and their sacred music in the old, showing that they understood that polyphony was proper to the mass and monody–the kind of music that has chords plus a single melody, like the result of guitars and singing–belonged to the world.
Throughout the 18th and 19th Centuries secular music (specifically opera) crept into the church more and more, although some composers continued to write in the old style to preserve it. In the second half of the 19th Century, the monks at Solesmes compiled the Liber Usualis and used it to start a revival of Gregorian Chant that extended into the beginning of the 20th Century. It inspired a new group of Catholic composers to write in the old style, using chant as an inspiration. Then as the century wore on, chant slowly fell back out of use along with polyphony. It’s been the case for a long time in the church–chant is used then lost then recovered. We need to recover it again, and the guitar is not part of the chant or polyphonic tradition. The organ, however, is at the heart of it.
I would encourage those of you that play the piano at church to move over to the organ. If you ignore the pedal keyboard, it’s not that big a jump at first, and you can make the transition slowly, starting at first with only the manuals and a few stops, and adding to your repertoire as you learn. I would also encourage you to re-discover chant. It is the proper music, and the reason behind the church condemning the use of other instruments in mass. Chant. Some of it is fiercely hard, so that there is always a challenge, but much of it is completely accessible, especially in this day of internet videos. There are instructional videos on youtube; it’s not that hard to learn.
This is a lot coming from someone who bleeds burnt orange! 
But it is not a totally one way street - it isn’t the case that one day they told all the composers - “we have decided polyphony is good, now you may compose the first polyphonic music.” The music was composed, it was new but seemed in keeping with the tradition, it worked in practice, the Church approved it (although they also at one point reeled in the complicated polyphony - a good example of them deciding the currant trend wasn’t working.)
I think properly tuned instruments are a good refence point for what notes to sing.