Pope Gregory I the Great dies. The first monk to become pope, Gregory was arguably the right man in the right place at the right time.
This not only protected Rome politically and militarily, but provided a source of income for the papacy, thus making it more independent and more powerful.
He also sends missionaries out to various pagan tribes, something which not only expands Christian belief but also the political influence of Christian leaders like himself.
Second, Pope Gregory increased the power of the papacy within the Church by decreasing the power of bishops. He did not specifically promote the doctrine of papal primacy, but he did create religious orders that are independent of the bishops and answerable directly to the pope.
He was also a strong proponent of clerical celibacy, something not yet widely accepted even in the western Christian Church. This will prevent clerics from passing down titles and lands to their children, thus further reducing the independent power of bishops.
Finally, Pope Gregory makes extensive changes to the liturgy of Christianity - the music used, the order and selection of readings from the Bible, and so forth.