For this reason, I bow my knees before the Father, from whom every family in heaven and on earth derives its name, that He would grant you, according to the riches of His glory, to be strengthened with power through His Spirit in the inner man; so that Christ may dwell in your hearts through faith; and that you, being rooted and grounded in love, may be able to comprehend with all the saints what is the breadth and length and height and depth, and to know the love of Christ which surpasses knowledge, that you may be filled up to all the fulness of God.
Eph 3:14-19 NASB
I find it interesting that after Paul had written quite a bit to the believing Ephesians about their solid identity in Christ, their acceptance in Christ, the life that they have in Christ, how they are partakers in Christ and the unfathomable riches that they have in Christ, in his prayer for them he prayed "that Christ may dwell in your hearts through faith."
I mean, from the very beginning, when they first believed, didn't Christ already come to dwell in their hearts? Wasn't it true that as Ezekiel foretold, in the New Covenant they would be given a new heart and a new spirit (Ez 36:26), and didn't Paul write elsewhere in various different ways that Christ already dwelled in them? So why would he then pray that Christ would dwell in their hearts by faith?
Well, it is true that as believers, we have become new creations in Christ and we have a new heart and a new spirit that is unlike the old heart and spirit that was replaced. Christ does dwell in us, and He will never leave us nor forsake us! (Heb 13:5) It's by grace, through faith that we died with Christ and are hidden in Him (Col 3:3) and we rose again with Him (Eph 2:6, Col 3:1) and we are one spirit with Him (1 Cor 6:17). These are done deals. They are always true of us at the core of who we are, in our spirit.
But there is a soulish realm that differs from the spirit realm. While our spirit doesn't change and remains fully alive and regenerated and filled with the life of Christ, and is born again once and for all, our soul is constantly being renewed and transformed. In the context of the passage that I started with, that is what the heart is. It's our mind, will and emotions. It's the part of us that thinks, believes, feels, understands, etc. (Example: Rom 10:9 says "Believe in your heart..."). Whereas our spirit was forever changed in a one-time event, our heart/soul is constantly going through changes.
Our spirit has been forever sanctified and established in Christ, and "born again of incorruptible seed" (1 Pet 1:23), and at the same time in the heart and soul realm we are constantly going through a renewal process. "It is good that the heart be established (or 'strengthened') in grace" (Heb 13:9). This verse shows that the believer's heart goes through a process of being established and strengthened. A little earlier in Hebrews, the writer encouraged the believers to "draw near with a true heart in full assurance of faith" (Heb 10:22). Do you see how it's possible for the heart to fluctuate, while all along the spirit remains as it was from the time we first believed! This is why Paul prays for the hearts of the Ephesians - that Christ may dwell there. There's no fear here of Christ ever actually leaving them, or not dwelling in their spirit, but Paul wants them to be assured and rooted and grounded in the truth and reality of it all. And how are they to be rooted, grounded and assured? By faith. That is, by having faith in Christ.
This isn't the faith of Christ. This is our faith in Christ. Faith is "conviction of the truth." (See here). To have faith is to be persuaded of the truth. We are exhorted to walk by (our) faith, not by (our) sight. To be convicted of the truth, and to be rooted and grounded in God's love, we walk by faith, which is "the substance of things hoped for, the evidence of things not seen" (Heb 11:1). Our circumstances in life (what we "see") - good or bad - do not generally give us a good picture of the reality of Christ, because He is not seen and known through what we see and experience! We know Him through what we do not see. He "dwells in our hearts," not through what we can see and experience in our daily circumstances, but through faith. That is, through believing the truth about Him whether our circumstances seem to be a good demonstration of the truth or not.
We already have all the love of God, all the grace of God, all the fullness of God and "every spiritual blessing in the heavenly places in Christ" (Eph 1:3). These things are our reality. Even though we can't "see" them, they are far more real than everything that we can see! And yet we are exhorted to grow in grace (2 Pet 3:18), and in the beginning passage Paul prayed that they may be "rooted and grounded in love" and "filled up to all the fullness of God." So is all of this contradictory? We have all the love, grace and fullness of God and yet we're to continue to be filled with it? No, it's not contradictory. These things simply refer to two different parts of who we are. The first is the spiritual realm; the second is the realm of the heart and soul.
We are filled, and yet we're to continue to be filled. We have all of God's grace and yet we're to grow in it. We have all of God's love and yet we're to continue to be rooted and grounded in it. So again, how does all of this work? How does all of this happen? By faith. By believing. By being continually, over and over again, convicted and convinced in our hearts of the truth of what is already true and real about us. It's about grasping and comprehending the truth. It's about repenting - that is, changing our minds - and believing the truth of the gospel. It's about renewing our minds day after day to the truth of the good news, so our hearts may be filled with the fullness of Christ.