The word therefore has two different thoughts on interpretation. Many pastors and people in general like to say “what is this there for?” and while that is true, it tends to minimize the impact of the word. We need to think of Therefore as a transition to form a cause and effect situation. In essence we are saying the reason for what is to come is because of what was just written about. In this situation the cause is the past life we lived where we were dead in our sins (see this blog entry from Friday) and now Paul takes that previous life we had and the raising of Christ to remind us.
Specifically Paul is reminding the Ephesians that they were the uncircumcised, which really is just a matter of Jews vs. non-Jews (Gentiles). For any unfamiliar with the debate, the Christians who were Jewish in lineage would require the Gentiles to follow the same rules they had been subject to during the time of the Old Testament. Paul then reminds these gentiles that prior to Christ, they were separated from God and unable to be saved. They had no idea of the covenants God made with Israel, but now they have been brought to God through the blood of Christ.
Then Paul reminds the church that Jesus demolished the differences between Jew and Gentile making it possible for Christians to be united in Christ regardless of lineage. This is why Christians ought to be the ones going to every single person in this world regardless of heritage, skin color, or any other factor to share the gospel with them for we are all one and the same in Jesus. This really goes with most scientific research showing the difference in humanity is virtually nothing in the grand scheme of our genes.
In Christ everyone is reconciled to God by the cross and that is why we can say the cross is the great equalizer. We are all equally guilty before God prior to the cross and equally redeemed after the cross. There is one cross, one God, one death, one redemption for all of mankind.
As a result of this, the gentiles are now part of the kingdom of God. We are adopted into his family and there are many illustrations of this throughout the Bible including examples of a branch being grafted in to a tree as given by Jesus. In this family, we have the whole of church history as part of our family including the Apostles and Prophets as well as every Christian today. In all of this, Christ is what joins us together as we are His church, His body, and all of us are saved by His death on the cross.