Paul wants his readers to follow an argument from the smaller to the greater. If Israel's fall brought so much good to the world, their restoration must bring even more. Barnes explains the first half: Israel's rejection, an event that seemed so unlikely to spread true religion, became the occasion of many blessings to mankind through the spread of the gospel. The question then invites the reader to imagine what Israel's "fulness" will do. It turns Paul's sober words about Israel's loss into hope.

The question answers itself: if loss brought riches, fulness will bring far more. Paul explains why he says this to Gentile readers: "I magnify mine office: If by any means I may provoke to emulation them which are my flesh, and might save some of them" (Romans 11:13-14). He then repeats the argument in a sharper form: "what shall the receiving of them be, but life from the dead?" (Romans 11:15). Later he states his hope plainly: "And so all Israel shall be saved" (Romans 11:26).

Gill explains that "the world" here means the Gentiles, as the next phrase shows. "Diminishing" translates hēttēma, which Barnes defines as "diminution, defect, that which is lacked or wanting." "Fulness" is plērōma, which Barnes says "means that which fills up, or completes anything." Set against the fall and the diminishing, he says it "evidently means their complete restoration to the favor of God." Gill notes that the few Jews who believed carried the gospel, "the unsearchable riches of Christ" (Ephesians 3:8), into the Gentile world.

This question reveals how God multiplies grace. Barnes explains that the gospel is called riches because it is "the means of our highest enjoyment and eternal welfare." Matthew Henry puts it simply: "The gospel is the greatest riches of every place where it is." God brought that wealth to the nations through Israel's loss.

If God can do so much through a fall, Paul reasons, His restoring work will be greater still. The question calls the reader to expect more from God's mercy, not less. Gill pictures that fulness as a time when converts among Israel "will be as the sand of the sea."