Medieval Christians witnessed the rediscovery of Aristotle and the Greek Philosophers. But not all Christians of the time period were learned.
According to the philosophers of the time, the purpose of life was to live according to ones nature. The purpose of a mouse was to become the best mouse within its means. The purpose of a human was to become the best human within his/her means. Mastery of the virtues was the key to becoming the best human you could be. The man who was most fully human, was also most virtuous, and consequently most complete.
On the flip side of the coin, the simple people who worked the fields were not reading Plato. The purpose of life, as they understood it, was simply to give glory to God. This is seen in the incredible detail with which they built their churches and cathedrals.
The nature of human life according to medieval thinkers accords with Augustine's philosophical statements that humans and the world were created by a perfectly good God and were given the ability to freely choose between good and bad thus driving themselves away from God when bad is chosen: bad, or evil, is the absence of good, which is a positive requiring an active participation and choice.