Only the human voice, as the conveyer of the Logos, is permitted a place in the Byzantine Divine Liturgy (like the "theatre of the inner word", the Liturgy must be free of distractions that impede the communication of the Logos), and likewise the Roman Church has been particularly conservative in its admission of any secular instrumentation. Like traditional Ruthenian Catholic churches where to this day only prostopinije plainsong is used, polyphony was only admitted in the Western Church with severe limits - the Council of Trent required that the words still be intelligible - and then only after Palestrina had demonstrated (in his Missae Papae Marcelli) that these requirements could be met. Vatican II's Constitution on the Sacred Liturgy, Sacrosanctum concilium, upheld Gregorian plainchant as the normative form of liturgical music, with the admission that Renaissance polyphony could be done in accord with the same spirit:
116. The Church acknowledges Gregorian chant as specially suited to the Roman liturgy: therefore, other things being equal, it should be given pride of place in liturgical services.
But other kinds of sacred music, especially polyphony, are by no means excluded from liturgical celebrations, so long as they accord with the spirit of the liturgical action, as laid down in Art. 30.Though the Church's teaching on instrumentation became universally ignored in the 1970s, the Western Church has also restricted secular instrumentation during the Liturgy, except for the organ. Tra le sollecitudini, a bull of Pope St. Pius X issued in 1903 and never abrogated, directs the following:
The employment of the piano is forbidden in church, as is also that of noisy or frivolous instruments such as drums, cymbals, bells and the like.
It is strictly forbidden to have bands play in church, and only in special cases with the consent of the Ordinary will it be permissible to admit wind instruments, limited in number, judiciously used, and proportioned to the size of the place-provided the composition and accompaniment be written in grave and suitable style, and conform in all respects to that proper to the organ.
The fact that pride of place is given to the organ seems on the face value to be particularly bizarre. Though there are some aspects in which the organ resembles the human voice - by virtue of conveying wind through pipes - the same is true of any wind instrument, While traditionally the organ is thought of as a purely sacred or liturgical instrument in the West, inherently associated in people's minds with church, this is a bizarre fact that needs to be explained.
It is bizarre because historically the organ accompanied circus acts in the Roman Empire - acts which in earlier days accompanied the martyrdom of Christians, and in later days were pornographic in character; the terms "actress" and "prostitute" were synonymous among the Byzantine Romans. The old Catholic Encyclopedia relates that "the fact remains that by the Fathers of both East and West all forms of the drama were banned indiscriminately and in terms of the severest reprobation," and the Council in Trullo - regarded as an Ecumenical Council by the Orthodox and Eastern Catholic Churches, though it was never promulgated in the West - banned actors and actresses from Communion. Even pagan priests under Julian the Apostate's regime were banned from attending the theatre. Consequently, that a musical instrument with this association should end up a liturgical instrument is simply bizarre.
Egon Wellesz gives an explanation in his History of Byzantine Music and Hymnography, pp. 97-98:
The use of the instrument in the Western Church may be explained in the following way. In 757 Constantine Copronymus sent an organ as a present to King Pippin. In 812 Michael I presented Charlemagne with another instrument. The gift was accompanied by musicians who knew how to play the organ, and who obviously taught their art to Frankish musicians. It is also reported that the instruments were copied by Frankish craftsmen and the new organs used to assist the teaching of Plainchant. Since all this work was done by the monks, it follows that the organ was gradually introduced inside the church and spread all over the West as a church instrument. Organs of a larger size were built, and the Byzantine portable organ was replaced by instruments of the size we know nowadays, one of the earliest being the great organ at Winchester, built in 980.
Relations between the Byzantines and the Franks were tense, to say the least, especially when the Franks started claiming the title of "Roman Emperor", since the Byzantines held territory in the West and had historical continuity with the real Roman Empire. Though Michael I gave lip service to Charlemagne as "basileus", one can imagine his gift of the organ being a tongue-in-cheek insult that went over the Frankish barbarian's head. Though this interpretation is just a guess on my part, if so, this insult gave rise to a great tradition in Western sacred music, which owes its patrimony to the fact that European culture was redeveloped by a Frankish invader free of any knowledge of the connotations that the organ had in Roman culture.