Early Music of the Christian Church.  Plainsong  

The earliest music of the Christian Church borrowed much from the Greeks and from the Jews, who have always been a music-loving people.  Singing was a part of church worship from the very beginning.  St Ambrose, bishop of Milan, made a collection of tunes in the fourth century, and in the sixth century the great pope, St Gregory, collected the large body of plainsong, which is known as Gregorian plainchant, and is still sung in churches today. 

But again our imagination has to be exercised when we realize that, before the ninth century, A.D., no one had thought of producing a chord.  Imagine a world without "harmony." of any kind!