Christian music is music that has been written to express either personal or a communal belief regarding Christian life and faith. Common themes of Christian music include praise, worship, penitence, and lament, and its forms vary widely across the world.Like other forms of music the creation, performance, significance, and even the definition of Christian music varies according to culture and social context. Christian music is composed and performed for many purposes, ranging from aesthetic pleasure, religious or ceremonial purposes, or as an entertainment product for the marketplace.Worship servicesAmong 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—, or both. 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. 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.