Paul speaks about the suffering that he continues to experience. He may be acknowledging here that there is much suffering that he and all other Christians must endure before Christ comes again, and that Christ in a spiritual sense joins with them in experiencing these hardships. Paul certainly does not mean that Christ's sufferings alone were not enough to provide salvation for the believers.
This means to bring about the purpose of God's gospel message, which is that it be preached and believed. "Word of God" here is a metonym for the message from God. AT: "to be obedient to what God has instructed" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])
Paul speaks of the value of this secret truth about God as if it were a treasure of material wealth. "riches" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Paul speaks of the believers as if they were actual containers in which Christ is present. This is one of his ways of expressing the union of the believers with Christ. (See: [[rc://en/ta/man/translate/figs-metaphor]])