# so that their hearts

"so that their hearts and yours" 

# brought together

This means brought together in a close relationship.

# all the riches of full assurance of understanding

Paul speaks of a person who is completely sure that the good news is true as though that person were rich in physical things. 

# the mystery of God

This is knowledge that can be revealed only by God.

# that is, Christ

Jesus Christ is the mystery revealed by God.