Groups or even whole nations easily get into hostility with each other. In Paul's time there was the religious divide of Jew and Gentile ... with a wall of hostility between them. The fact is both sides also have a barrier between themselves with God ... and life without God leaves us open to develop hatred of one another.
But Jesus came to remove that barrier between us and God, and he did it for any and everybody. With the barrier gone, everyone is free to become God's friend. With that hostility we also become free to relate to other people - people on the other wide of our own walls. These melt away, and are replaced by blessing.
With such divides gone we are free to work together. We are knit together purposefully in Jesus Christ. A building analogy is used: Jesus sets the parameters, being the chief cornerstone. The rest of the foundations are found in the apostles and prophets (interesting - not in rules and regulations!).
What is built as a result? Not big stone edifices, or listed buildings to stand for evermore ... but groups of people working together in Christ. They become the 'temple', where God resides, can be found, and is worshipped. Every person has a part to play, and can play their part.
You too can be included!
Yet we are there not just to look nice. Remember back to last week and v10 - God has work for us to do!