Instruments weren't introduced into churches until about 670 AD. Before that, churches relied on chanting to worship with music. The most popular form of chanting was Gregorian chanting, developed by Pope Gregory I. It originated in the third century AD.
Barber Shop Singing
Barber Shop singing groups are comprised of four men singing four different parts without instruments. The lowest part is the bass, then the baritone sings a harmony part above the bass, then the tenor above the baritone, then the lead, who sings the melody. Barber shop music became wildly popular in the 1940s. Popular barber shop quartets include the Dapper Dans of Disneyland and the Buffalo Bills, who appeared in The Music Man.
High School and Collegiate A Cappella
The longest standing collegiate a cappella group is the Yale University Whiffenpoofs, founded in 1909. Since the "Whiffs" were founded, collegiate a cappella groups have popped up around the world and grown exponentially, said Deke Sharon,founder of The Contemporary A Cappella Society.
According to Sharon:
High school groups have since followed suit. Sharon said he wasn't sure how many exist, but did say that more and more traditional high school choirs have moved in the pop music direction since the early 2000s.