Thus is his prayer now again, the very same as when he began. For what were his words in the beginning? "That the God of our Lord Jesus Christ, the Father of glory may give unto you a Spirit of wisdom and revelation in the knowledge of Him; having the eyes of your heart enlightened, that you may know what is the hope of His calling, what the riches of the glory of His inheritance in the saints; and what the exceeding greatness of His power to us-ward who believe." And now again he says the same. "That ye may be strong to apprehend with all the saints what is the breadth, and length, and height, and depth;" i.e., to know perfectly the mystery which has been providentially ordered in our behalf: "and the breadth, and length, and height, and depth;" that is, too, the immensity of the love of God, and how it extends every where. And he outlines it by the visible dimensions of solid bodies, pointing as it were to a man. He comprehends the upper and under and sides. I have thus spoken indeed, he would say, yet is it not for any words of mine to teach you these things; that must be the work of the Holy Spirit. "By His might," says he, is it that you must be "strengthened" against the trials that await you, and in order to remain unshaken; so that there is no other way to be strengthened but by the Holy Ghost, both on account of trials and carnal reasonings.
But how does Christ dwell in the hearts? Hear what Christ Himself says, "I and my Father will come unto him, and make our abode with him." [John 14:23] He dwells in those hearts that are faithful, in those that are "rooted" in His love, those that remain firm and unshaken.
"That ye may be" thoroughly "strong," says he; so that there is great strength needed.
"That ye may be filled unto all the fullness of God."
What he means is this. Although the love of Christ lies above the reach of all human knowledge, yet shall you know it, if you shall have Christ dwelling in you, yea, not only shall know from Him this, but shall even "be filled unto all the fullness of God;" meaning by the "fullness of God," either the knowledge how God is worshipped in the Father, the Son, and the Holy Ghost, or else urging them thus to use every effort, in order to be filled with all virtue, of which God is full.
The one who knows the love that “passes all understanding” will better express the full measure of love for Christ. Paul prays that they may first know [the love of Christ] rather than do something. Doing comes from this knowing.
Can any words adequately describe this mystery, that God is born as a man? That God dies for the human race, the master for his servants, the Creator for his creation, the righteous for the unrighteous?… In the greatness of his majesty he became lowly to do on our behalf what was worthy of his love, so that we, insofar as we can, should join his household.
Faith is never directed solely to the Father or solely to the Son [but to Father and Son in their relationship]. Hence he adds “that you may be filled with all the fullness of God.” Thus by confessing Christ and giving thanks to him in the same glorious terms, the same honor is reserved for the Son as for the Father. All things that come from God the Father have been restored through his Son. By this the faithful confess the divine perfection in its wholeness.
[AD 407] John Chrysostom on Ephesians 3:18-19
But how does Christ dwell in the hearts? Hear what Christ Himself says, "I and my Father will come unto him, and make our abode with him." [John 14:23] He dwells in those hearts that are faithful, in those that are "rooted" in His love, those that remain firm and unshaken.
"That ye may be" thoroughly "strong," says he; so that there is great strength needed.
"That ye may be filled unto all the fullness of God."
What he means is this. Although the love of Christ lies above the reach of all human knowledge, yet shall you know it, if you shall have Christ dwelling in you, yea, not only shall know from Him this, but shall even "be filled unto all the fullness of God;" meaning by the "fullness of God," either the knowledge how God is worshipped in the Father, the Son, and the Holy Ghost, or else urging them thus to use every effort, in order to be filled with all virtue, of which God is full.