This statement shows a total lack of understanding of the role of the organ as an accompanist and came out of a discussion which dismissed chant as being impossible to sing unaccompanied since a "congregation cannot sing the Pange Lingua without going flat."

If a congregation sinks to a more comfortable pitch when you are chanting, then possibly the problem is the pitch that you choose, not the limitations of the congregation. This applies if the pitch rises as well. And...really, who cares if the pitch changes?

But that inspired a guitarist to jump in and complain about composers who insist that their songs appear without chords....so he respects that request by writing in chards and playing them anyhow.

The idea that a guitar can adequately support singing of a choir and congregation is as weak as the idea that a piano can replace an organ.

Pitches

The guitar has a limited range of pitches it can play. It cannot play as low in pitch as the piano does and the piano cannot play as low in pitch as the organ can.

A guitarist can only play 6 pitches simultaneously. A pianist 10. An organist can, on an average-size instrument, play 132.

The guitarist can play a chord up an octave or down an octave, but not at the same time. The piano can play melodies in octaves in either hand, but like the chords on an guitar, can only play chords up or down an octave. An organist can play a chord at normal pitch, plus up an octave, another octave, another octave, an octave and a fifth, another octave, two octaves and a third…and more.

Tone Color

The guitar, depending upon the point on the string that is plucked, has a limited palette of tone colors, but more than a harpsichord, which only has one per string set, plus one when a buff is used. The piano tonal palette relies upon touch to modify the tone color of the note played, along with the una corda pedal and use of the sustain pedal. But a piano is never confused with possibly being another instrument except for the harp.

The organ has at least 4 distinctly different tone colors with many variations within them.

Voice Leading

Guitarists, unless extremely talented, frequently play parallel fifths and fourths when accompanying. Pianists and organists rarely, if ever, play parallel fifths and fourths when accompanying church music. Pianists and organists can easily play melodic lines against each other that provide a subtle harmonic support and direction for sung melodies.

The organ's ability to play in octaves permit a bass line to be doubled an octave or even two octaves below, but more importantly, pitches above the melody in octaves or other pitches of the harmonic series can be played to make it easier for the singer to hear their own pitches when singing. Using the organ in this manner more fully supports a singer/singers. The only practical option for a guitarist or pianist is to play louder.

Sustain

The sound of the guitar and piano begins to die shortly after the strings are plucked except in the finest instruments, which bloom in sound before dying. The sound of an organ is constant as long as the key is being depressed. Singers are driven forward or held back by the percussive sound of the guitar or piano. The organ can be played in a "choppy" style to lead a congregation or singer, but most often it leads by the flow of the changing harmonies of the music that is played
Resonance

The guitar resonates from the ability of the wooden box to amplify the loudness of the strings. The piano sound is limited to the ability of the soundboard to resonate within the case. The lid on a piano may be raised to permit more of this sound to be heard as it is reflected by the lid out into the room. The organ is designed to use the room itself to resonate, filling the entire room with sound, no matter whether the organ is playing softly or at full volume

A church organ is built to fill a room with sound. A piano is built to be listened to in a room, as is the guitar.

The organ is not the choice of the church because it sounds more "holy" than the guitar or piano, but because it fulfills the role of accompanist more fully than any other instrument except the symphony orchestra.

The guitar and piano are both instruments that can play music of great beauty, but putting them into the role of accompanying church music as we know it limits the singing in many ways including eliminating much music that can only be played on the organ, great church music that cannot be played on any other instrument.