Christianity being a foundation of Christ it can be referred to the continuity of Christís mission. Before ascension he commanded his disciples to go and proclaim the word of God to the rest of the world.
To fulfill this early churches were established, this were to enable effective preaching to large number of people who would expand Godís word.
They were to preach of repentance of sin, true new life in Christ with everlasting eternity and retrieve the lost to God.
What have you yourself done or seen other people do to fulfill this?