I'm not a musicologist. While I know a fair amount about music in some respects, the musical tradition is vast, so this post may reflect gaps in my knowledge.
1. A striking feature of the Christian faith is the centrality of music in worship, as well as the variety of musical styles and performance styles. Christianity is a religion set to song. This has its roots in the Psalter.
Qui canit bis orat ("he who sings prays twice") is a saying attributed to Augustine. There's a variation: qui bene cantat bis orat ("he who sings well prays twice").
Apparently, Augustine never said that, but it seems to be a summary of something he did say. According to the saying, when you sing a sacred text rather than speaking a sacred text, that's like praying twice. You sing once but pray twice, because it's like a prayer set to music.
And it's true that setting a sacred text to music can deepen it in the experience of the singer. That makes it more memorable, and adds an emotional dimension, because music reflects or evokes a mood in the mind and heart of the listener.
There are, of course, churchgoers who sing mechanically with no faith. So there's a difference between mere singing and singing well. Not well in the sense of execution but attitude.
2. Different traditions within Christendom, or at different periods, have different musical ideals. In traditional Catholicism, there's the ideal of male choirs singing a cappella music, viz. Gregorian chant, Palestrina. "Sexless" music that approximates an angelic ideal. Mixed choirs were forbidden. For a long time the Sistine chapel used castrati.
Nuns could sing in convents. Vivaldi composed music for a girl choir, but not for performance in church.
In the Anglican tradition you have male choirs consisting of basses, tenors, countertenors, and trebles. German choirs used boy sopranos and boy altos. I don't know the details of the French tradition.
However, there may be a historic distinction between the composition of a parish choir and a cathedral choir. In addition, there's Russian Orthodox choir music.
3. A problem with the sexless ideal is that humans aren't angels. We're earthy, embodied, gendered beings. It's natural for vocally well-endowed men to have virile voices and vocally well-endowed women to have sensual voices. That's not a matter of flaunting one's sexuality. But it's nothing to suppress or be ashamed of.
The voices of boys and girls have different timbres. As such, it's a bad idea of have mixed choirs of boys and girls. Blending the timbres obliterates the distinctive beauty of each. Rather, there should be boy choirs and girl choirs. I'm referring to classical vocalism, where boys and girls use the head register.
Although I prefer the timbre of English boy choirs to German boy choirs, I prefer the German tradition of using boy altos rather than countertenors. The countertenor is an artificial voice that lacks the natural appeal of a boy alto or the natural of female alto. In addition, it's a magnet for sodomites, which puts choirboys at risk.
It's good that we now have mixed choirs of adult men and women. When women carry the soprano line in choral music, they have warmer, fuller voices than boys. Greater tonal and emotional weight. That's a particular virtue in music on a larger scale.
Likewise, women are often superior as soloists. There are concert sopranos like Emma Kirkby who cultivate a straight, boyish tone, but that's not what I'm looking for in a female voice. I listen to a woman for a voice with feminine allure.
Boy's voices used to break at a later age. For instance, Ernest Lough's voice didn't break until he was 17! That may account for his fuller tone and vibrato. He had a mixed voice: not a pure bell-like head register, but a mix of head and chest registers.
Church music should exploit the natural goods of different kinds of voices, by sex and age: men, women, boys, and girls.
4. To my knowledge, there are about three factors that contributed to mixed choirs and female soloists:
i) Part singing
Unison singing lacks the same range as part singing. As music became more complex, making greater demands on vocal range, that created a niche for female sopranos, although trembles could occupy the same niche.
ii) Congregational singing
Protestants stressed the value of congregational singing, which broke down gender barriers in worship with regard to singing.
iii) Opera
Opera fostered many roles for women. Although some roles were taken by castrati, there were early divas like Elizabeth Billington, Lucrezia Aguiari, and Angelica Catalani who were very popular. The opera stage didn't have the same restrictions on female participation as the church. But that, in turn, began to break down barriers.
Handel used women from his operas to sing in his oratorios. Although that was initially performed outside the walls of the church, it was a difficult dichotomy to maintain.
5. Thus far I've focussed on the classical musical tradition, but church music has expanded far beyond that. There's Southern folk hymnody, Latino church music, and the black musical tradition, beginning with "Negro" spirituals, followed by traditional Black Gospel music, with its own divas (e.g. Bessie Griffin, Mahalia Jackson, Inez Andrews, Marion Williams). Not to mention the country-western tradition–along with crossover artists (e.g. Johnny Cash).
