Much ink has been spilled over what, exactly, Paul means in Ephesians 3:18 when he speaks of the “width and length and depth and height.” To begin with, the phrase is missing an object – the width and length and depth and height of what? Then, there is the perennial temptation to allegorize. What might Paul mean by each dimension individually?
Augustine, for example, equates the width and length and depth and height with the virtues of love, hope, patience, and humility, respectively? Matthew Henry is perhaps more convincing, but equally allegorical, when he suggests that
By the breadth of it we may understand the extent of it to all ages, nations, and ranks of men; by the length of it, its continuance from everlasting to everlasting; by the depth of it, its stooping to the lowest condition, with a design to relieve and save those who have sunk into the depths of sin and misery; by its height, its entitling and raising us up to the heavenly happiness and glory.
However, most commentators see the width and length and depth and height as referring to “the love of Christ which passes knowledge,” with the individual dimensions “interpreted together in a collective sense to express the immensity of the subject.” Thus Calvin, with characteristic confidence, writes that
what follows is sufficiently clear in itself, but has hitherto been darkened by a variety of interpretations…. By those dimensions Paul means nothing else than the love of Christ, of which he speaks afterwards.
However, having established that the width and length and depth and height refer to the love of Christ, and that the individual dimensions do not necessarily have any hidden allegorical meaning, the use of this phrase in one of the great missionary chapters of the Bible is not insignificant. Paul begins his prayer in Ephesians 3:1 and is immediately distracted. As soon as he mentions the word “Gentiles” he cannot help but launch into a meditation on what it means that he, a Jew, is addressing the Gentiles as fellow heirs of the grace of God. The mystery, as Paul explains in Ephesians 3:6, is “that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel.” In other words, the “mystery” is nothing less than the worldwide missions movement which began with Paul to the Gentiles, and has continued in successive generations ever since!
Paul refers to the outpouring of the grace of God to the Gentiles as a mystery, not because it was not known, but because it was not previously known. Thus, knowing becomes a theme for Paul in this section. The mystery was made known to Paul in verse 3, that we might understand his knowledge in verse 4, which was not made known in other ages in verse 5. Paul preached to make all men see the fellowship of the mystery in verse 9, so that the manifold wisdom of God might be made known even to the principalities and powers in the heavenly places in verse 10. This language culminates in the verses that we have been considering – Paul’s prayer that we might be able to comprehend the width and length and depth and height, to know the love of Christ which surpasses knowledge.
So, I won’t go so far as to say that the breadth of God’s love (or any other dimension in particular) refers specifically to His plan to pour out His grace upon the Gentiles. Nevertheless, the expansiveness of the love of God is manifested in the expansiveness of the Gospel to all nations. As westerners, we often read this verse individualistically – as referring to the width and length and depth and height of Christ’s love for us as individuals. But clearly, if we follow the progression of Ephesians 3, part of comprehending the width and length and depth and height of the love of Christ is to understand the mystery of the inclusion of the Gentiles into the covenant of grace made with Abraham. We will never understand the love of Christ toward us, until we understand the love of Christ toward the nations. To know the fullness and extent of the love of Christ, we must have a heart for missions.
Ephesians 3:1–21 (NKJV)
The Mystery Revealed
3 For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles—2 if indeed you have heard of the dispensation of the grace of God which was given to me for you, 3 how that by revelation He made known to me the mystery (as I have briefly written already, 4 by which, when you read, you may understand my knowledge in the mystery of Christ), 5 which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: 6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel, 7 of which I became a minister according to the gift of the grace of God given to me by the effective working of His power.
Purpose of the Mystery
8 To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, 9 and to make all see what is the fellowship of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ; 10 to the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places, 11 according to the eternal purpose which He accomplished in Christ Jesus our Lord, 12 in whom we have boldness and access with confidence through faith in Him. 13 Therefore I ask that you do not lose heart at my tribulations for you, which is your glory.
Appreciation of the Mystery
14 For this reason I bow my knees to the Father of our Lord Jesus Christ, 15 from whom the whole family in heaven and earth is named, 16 that He would grant you, according to the riches of His glory, to be strengthened with might through His Spirit in the inner man, 17 that Christ may dwell in your hearts through faith; that you, being rooted and grounded in love, 18 may be able to comprehend with all the saints what is the width and length and depth and height—19 to know the love of Christ which passes knowledge; that you may be filled with all the fullness of God.
20 Now to Him who is able to do exceedingly abundantly above all that we ask or think, according to the power that works in us, 21 to Him be glory in the church by Christ Jesus to all generations, forever and ever. Amen.
 John Calvin and William Pringle, Commentaries on the Epistles of Paul to the Galatians and Ephesians (Bellingham, WA: Logos Bible Software, 2010), 263.
 Matthew Henry, Matthew Henry’s Commentary on the Whole Bible: Complete and Unabridged in One Volume (Peabody: Hendrickson, 1994), 2312.
 Glenn Graham, An Exegetical Summary of Ephesians, 2nd ed. (Dallas, TX: SIL International, 2008), 256-257.
 Calvin and Pringle, Commentaries on the Epistles of Paul to the Galatians and Ephesians, 263-264.