Ephesians is about the triumph of Christ over supernatural forces and how he is bringing unity to all things. One of the key passages for understanding this is Ephesians 4. Ephesians 4:1-6 bases an encouragement to unity on a Trinitarian reflection (One Spirit, One Lord, One God and Father). Then verses 7-16 shift the focus to diversity within unity.
It is in this second half that Paul gives one of the more detailed portraits of how Christ’s triumph and unity relate. He portrays Christ as a divine warrior using Psalm 68 who both ascends with captives (to fill all things) and also descends. Related to this spatial movement of ascension is his giving of gifts for the sake of unity. He gives apostles, prophets, evangelists, pastors and teachers.
So Paul connects Christ’s divine conquering to the gift of unity.
But Paul’s use of the Old Testament has troubled modern interpreters. A few have called this use of the OT as “very odd” while Fitzmyer even says that Paul completely disregards the original context of the Psalm.
In Ephesians 4:8 Paul quotes from Psalm 68:18, but he changes a crucial word. In the LXX it says that God “received” gifts from men. Paul instead quotes it saying that Christ “gave” gifts to men. Here is a chart showing that difference and the other changes Paul makes in comparison with the LXX. (I have taken this chart from Richard Lucas’ dissertation.)
So what are we to do with this change? In my study of the passage I have identified five main views which I put into a chart (the linked chart is in a PDF so it is easier to read). This chart is not exhaustive, but it does represent the major views that I have encountered. I hope this can serve interpreters so that they can more quickly get into the process of judging the benefits of each view instead of searching for the different views out there.
This is a difficult issue, but I think the first option is the most likely. Paul was reading the Psalm in light of the movement of the entire Psalm. He is not simply quoting one verse in abstraction from the rest of the Psalm but appropriating the narrative movement of the Psalm. This could be labeled creative interpretation, but it is also faithful interpretation. He looks back on Yahweh as the Divine Conqueror and sees the Psalm fulfilled in Jesus Christ. Christ is the victorious Divine Warrior who has the right to give gifts to his people because of his triumph.
For one of the better articles on this issue see Gombis’ article “Cosmic Lordship and Divine Gift-Giving: Psalm 68 in Ephesians 4:8.”