Large mixed choirs have been introduced into churches quite recently; in the beginning, however, the “professional” singer was the sole psalmist, and out of the group of psalmists was the schola formed at a later stage of development. Also, between the 11th and the 18th centuries the polyphonic pieces were performed by a group of 6 to 8 singers.
After the Council, when in many churches the chant was given over entirely to the congregations, very little has been done for bringing them up to their double role: to sing what is genuinely their own part, and to sing in some way the parts that earlier belonged to the professional singers. As a result, in current practice active participation does not mean that the congregation sings the liturgy, but it sings something during the liturgy, in other words, replaces the chants of the liturgy by ready-made or recently composed strophic songs. Do we remember the words of the Council? “The texts intended to be sung… should be drawn chiefly from Holy Writ and from liturgical sources.” What people actually sing has nothing to do with the text written in the Missal or other liturgical books. Much of the congregational repertory is badly made music: insignificant or quite horrible settings of words, which remain mostly on the same low theological and poetical level. When the Missal allows us to substitute appropriate chants for the Proprium, it requires that these chants must be of the same content as the official liturgical pieces. Nothing of this requirement has been fulfilled. The real intention of the Council has been negated by a presumed aim.
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)