My pastor has given Adrian and me nine sermons on marriage to listen to; it's a series by Tim Keller, PCA pastor from New York City. We've been trying to listen to one or two a week, and then discuss them over the phone. I highly recommend them, and I think I speak for Adrian too when I say that :-).
Tim Keller defines deep marriage oneness as coming from the process of two people journeying to a common horizon. For the Christian, this common horizon is heaven, and the journey together sanctifies. The whole series just oozes grace, but I especially like what Keller has to say about the sanctifying effects of marriage. He talks about how focusing on this common horizon and on sanctification can give a married person a vision of what God wants his spouse to become, and what they will be someday without the shackles of sin. And we should want others to catch that same glimpse of the beauty of this person.
Keller talks about the "glory self" and the "dross" in a person. The glory self is the person deep inside, the potential that God will bring to fruition in glorification. We get glimpses of this glory self in others sometimes, just as we get glimpses of a covered mountain when a wind blows away the clouds of fog for a brief moment. The dross in a person is the sin, the filth that God will slowly burn away through trials - and one of the ways He purifies people in this is by marriage. Marriage is a very sanctifying experience.
Now comes the part of one of his sermons that particularly struck both Adrian and me. Keller is speaking about dross and says that a non-Christian (or any spouse that is not Christ-centered, since Christians are not immune to self-centeredness) will look at the dross, the filth in his spouse and become disillusioned and discontented by his spouse, and say "I can envision someone better." This is why marriages end; one spouse wants to seek for someone who is better.
A Christian should look at his spouse, and not ignore the dross blindly. And he should say, like the non-Christian, "I can envision someone better." But the Christian should be envisioning his spouse as that "better" person, purified and sanctified; he should want the perfection that his spouse will become - the glory self that has been glimpsed - not chase after someone else's perfection. And he has the privilege of helping his spouse grow into that person. Wow. That is the promise, the beauty of the Gospel.