Among the most prevalent uses of Christian music are in church worship or other gatherings. Most Christian music involves singing, whether by the whole congregation (assembly), or by a specialized subgroup—such as a soloist, duet, trio, quartet, madrigal, choir, or worship band— . It is frequently accompanied by instruments, but some denominations (such as some Exclusive Brethren, the Churches of Christ, the Primitive Baptists and the Free Church of Scotland) or congregations still prefer unaccompanied or a cappella singing. Some groups, such as the Bruderhof, sing songs both with religious and non-religious meanings and words. For them, the act of singing is important. One of the earliest forms of worship music in the church was the Gregorian chant. Pope Gregory I, while not the inventor of chant, was acknowledged as the first person to order such music in the church, hinting the name "Gregorian" chant. The chant reform took place around 590–604 CE (reign of Pope Gregory I) (Kamien, pg. 65–67). The Gregorian chant was known for its very monophonic sound. Believing that complexity had a tendency to create cacophony, which ruined the music, Gregory I kept things very simple with the chant.
A page (leaf 12 recto
) from Beethoven's manuscript. Catholic monks developed the first forms of modern Western musical notation in order to standardize liturgy throughout the worldwide Church,
and an enormous body of religious music has been composed for it through the ages. This led directly to the emergence and development of European classical music, and its many derivatives. The Baroque
style, which encompassed music, art, and architecture, was particularly encouraged by the post-Reformation Catholic Church as such forms offered a means of religious expression that was stirring and emotional, intended to stimulate religious fervor.
In the West, the majority of Christian denominations use instruments such as an organ, piano, electronic keyboard, guitar, or other accompaniment, and occasionally by a band or orchestra, to accompany the singing. But some churches have historically not used instruments, citing their absence from the New Testament. During the last century or so several of these groups have revised this stance.
The singing of the Eastern Orthodox is also generally unaccompanied, though in the United States organs are sometimes used as a result of Western influence.
Some worship music may be unsung, simply instrumental. During the Baroque period in Europe, the chorale prelude (for organ) was widely used, generally composed by using a popular hymn tune thematically, and a wide corpus of other solo organ music began to develop across Europe. Some of the most well-known exponents of such organ compositions include Johann Sebastian Bach, Dieterich Buxtehude, George Frideric Handel, François Couperin, César Franck and Charles-Marie Widor to name a few. Up to the present time, various composers have written instrumental (often organ) music as acts of worship, including well known organ repertoire by composers like Olivier Messiaen, Louis Vierne, Maurice Duruflé, and Jean Langlais.
The church sonata (for orchestra and chamber group) and other sacred instrumental musical forms also developed from the Baroque period onwards.