In whom you also have heard the word of truth: the gospel of your salvation by which, having believed, you were sealed by the promised Holy Spirit who is the down payment of our allotment, toward the portion's redemption, to the praise of His glory.Interpretation
It is important to note how this passage relates to what proceeds it. The switch from speaking in the first person plural ('we') to the second person plural here ('you') emphasizes that before this point Paul was not talking about the Ephesians. Thus, all of the glorious inheritance that was being talked about before this only belonged to Paul and some group that he is a member of. Based on the rest of the context, this can be shown to be the Jews.
As such, what Paul has essentially been saying is that God as given the Jews this glorious inheritance. However, in this passage he adds that now we (i.e. the Gentiles) have now been added to this inheritance because we heard the gospel, and believed it.
The most beautiful part of the passage, to me, is the description of the Holy Spirit as our down payment. Our inheritance is being completely redeemed, but for now, we have first received the Holy Spirit which proves that our inheritance is coming, and He is also part of that inheritance: the first part.
What is most wonderful about this is that on this side of heaven we have a taste of eternity. The Spirit brings a bit of the eternal into our hearts and minds and now, right now, we beginning living out our eternal blessing.
It is bad theology to believe that we just try to make it through this life to experience the joys of God in the next. As Christians we do not to experience what God has to offer. Sure, we do not receive the fullness of it here, but we do receive it in part, and what we receive we can rejoice in and live in it.
Therefore, pursue the things of God. The riches of the kingdom of heaven are at our disposal by the presence of His Spirit. Let's take advantage of that!