The purpose of your life is to image God, to be in relationship with him and bring Him glory through enjoying Him. And in the Sermon On The Mount, Jesus is speaking to his disciples and the crowd and introduces two metaphors to create a picture of what our earthly lives are meant for. Jesus says his followers were created to be salt and light. We glorify God by being salt and light in this rotten, dark world. Our lives were made to be a means of preserving and illuminating the world with the light of Christ and bring glory to our Father.
The church is the body of Christ, the family of God. The church universal is every person that has been reconciled to God through saving faith in Jesus. And the church has been commissioned with a task, namely, to make disciples of all nations, baptizing them in the Name of the Father and of the Son and of the Holy Spirit, teaching them to obey all that Jesus has commanded us. That is our mission. And it comes straight from the Lord to us.