Our purpose was given by Christ himself. He gave these instructions in Luke 24:47, Matthew 28:19 and Matthew 16:18,19.
And that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning in Jerusalem
Luke 24:47 (NASB)Jesus came and told his disciples, "I have been given complete authority in heaven and on earth. Therefore, go and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit. Teach these new disciples to obey all the commands I have given you. And be sure of this: I am with you always, even to the end of the age.
Matt 28:18-20 NLTNow I say to you that you are Peter, and upon this rock I will build my church, and all the powers of hell will not conquer it. And I will give you the keys of the Kingdom of Heaven. Whatever you forbid on earth will be forbidden in heaven, and whatever you permit on earth will be permitted in heaven."
Matt 16:18-20 NLT