The main challenging concept that Lewis presents is this:
Even for their own sakes the loves must submit to be second things if they are to remain the things they want to be. In this yoke lies their true freedom; they are "taller when they bow." For when God rules in a human heart, though He may sometimes have to remove certain of its native authorities altogether, He often continues others in their offices and, by subjecting their authority to His, gives it for the first time a firm basis. (The Four Loves)Essentially, Lewis argues that all natural loves must be submitted to the higher love of God, that they must in a sense be killed, if they are to be preserved at all and made into something holy.
This subjection of love has to happen in Till We Have Faces as well (warning: spoilers!). The narrator and main character, Orual, suffers the loss of her beloved sister, Psyche, who is "offered" to a god whose existence Orual refuses to accept (the novel's a retelling of the classic myth of Cupid and Psyche). Orual, faced even with strong evidence that the god is there and is good, rather than joining her sister in worship, becomes distant and violently jealous. She fails to submit a natural love of her sister to the ultimate divine love.
In Vanauken's memoir, A Severe Mercy, he tells of his own journey through refusal to submit natural love to God's love and into submission in the end (warning: more spoilers!). When his wife, Davy, becomes a Christian, she begins to devote herself fully to Christ, and Sheldon, much like Orual at first, becomes jealous and cannot find his way to join her in this full devotion and obedience. His love for his wife takes first place in his life, stealing that "throne" in his heart, so to speak, from its rightful owner, Christ.
Both books tell of their main characters' coming to an understanding, as the result of extremely difficult circumstances involving suffering (and even death) of their earthly beloveds, that they must subject their natural loves to the highest love. Orual finally recognizes that through full obedience to the god, her sister has been made "a thousand times more her very self than she had been before the Offering" (Till We Have Faces). Yet still, the ultimate purpose and highest devotion, Orual learns, is to be for the god himself: "if [Psyche] counted (and oh, gloriously she did) it was for another's sake." In Vanauken's case, the literal death of his wife becomes the representation to him of the figurative death that any earthly love must undergo in order to be sanctified, or put in its proper place and thus ultimately exalted, under the authority of God's love. He proclaims near the end of the memoir: "If God saved our love - and, indeed, transformed it into its real and eternal self - in the only way possible, her death, it was for me, despite grief and aloneness, worth it."
And that's the key and the hopeful part of this challenging idea: Even though earthly loves must be subjected (in Luke 14:26 Jesus says we must even "hate" our families and very lives for His sake), ultimately by subjecting them God can transform them into their true selves. The earthly loves can be made even better, fuller, richer, when they are put in their proper places in submission to God.
What does this mean for me? My tendencies lead me to devote much of my attention and affection to my family, my husband, my earthly loves. In and of itself, such devotion is not bad, and God of course commands us to love one another. But my devotion to earthly loves can easily take the place of my devotion and obedience to Christ as the ultimate priority. For instance, when I'm feeling overwhelmed or depressed, I can lean on my husband for support, but ultimately shouldn't I rely fully on Christ for every need? Shouldn't I pray first rather than as an afterthought? Instead of fearing with great dread any time I perceive an earthly good might change or be taken away, shouldn't I acknowledge always that all earthly relationships are gifts from His hand, able to be removed from me? And shouldn't I be willing to accept any such removal without resentment or fear?
This submission is hard (to put it mildly), but I need to remember to cling to Christ alone, who will never fail or change, and to hold openly every earthly love. Hopefully then those earthly loves will be freed to blossom into holy and beautiful loves resting properly in my heart as second things.