Jamieson, Fausset and Brown on Ephesians 3:18

18. May be able--even still further. Greek, "May be fully able."

breadth ... length ... depth ... height--namely, the full dimensions of the spiritual temple, answering to "the fulness of God" (Ep 3:19), to which the Church, according to its capacity, ought to correspond (compare Ep 4:10, 13) as to "the fulness of Christ." The "breadth" implies Christ's world-wide love, embracing all men: the "length," its being extended through all ages (Ep 3:21); the "depth," its profound wisdom which no creature can fathom (Ro 11:33); the "height," its being beyond the reach of any foe to deprive us of (Ep 4:8) [Bengel]. I prefer to understand "the breadth," &c., to refer to the whole of the vast mystery of free salvation in Christ for all, Gentile and Jew alike, of which Paul had been speaking (Ep 3:3-9), and of which he now prays they may have a fuller comprehension. As subsidiary to this, and the most essential part of it, he adds, "and to know the love of Christ" (Ep 3:19). Grotius understands depth and height of God's goodness raising us from the lowest depression to the greatest height.