Who was the key figure in the spread of Christianity in the early medieval period?

The key figure in the spread of Christianity in the early medieval period is Augustine of Canterbury. Sent as a missionary by Pope Gregory I in 596 AD, Augustine is credited with bringing Christianity to the Anglo-Saxons in England. His efforts established the Christian church in England and helped convert various tribes, thus significantly expanding the reach of Christianity during this time.

Augustine's mission was pivotal not only for its immediate impact on the individuals he converted but also for the longer-term foundation it laid for Christianity's growth in the British Isles. His work led to the establishment of important ecclesiastical structures and set the stage for further missionary activities throughout Europe, making his role central to the Christianization of England and illustrating the broader spread of Christianity during the early medieval period.

The other figures mentioned played important roles in different contexts: Pope Gregory I was instrumental in the papacy's role during this period and in the promotion of Christian doctrine, Charlemagne contributed to the Christian empire by enforcing Christianity in his conquests, and Aquinas is known for his theological writings in a later period of the medieval era. While significant in their own rights, their contributions do not compare to Augustine's direct involvement in the initial spread of Christianity in the early medieval context.