In his new book Twentieth-Century Catholic Theologians, the Dominican Fergus Kerr questions the wisdom of moving the nuptial meaning of the body, which builds on the complementarity of the sexes, from the periphery to the center of theological speculation about the nature of God. This invites another question: How central to the Christian understanding of the meaning of marriage is the sexual difference between men and women? It is this question that Christopher Roberts addresses in his Creation and Covenant: The Significance of Sexual Difference in the Moral Theology of Marriage, and no one paying attention to the arguments about the blessing of same-sex unions in the Christian churches will want to ignore it. Roberts says he aims to raise the level of the theological conversation now dominated by questions of the justice of treating heterosexuals and homosexuals equally. Have most Christian thinkers thought sexual difference to be morally and theologically important? If so, does the contemporary discussion take account of their insights and arguments?

Interrogating the tradition in this way requires Roberts to do some detective work, since the question was not always asked in quite the sharp and insistent way we ask it today. So he must proceed by indirection, especially in the early centuries. He takes us from the second-century apologist Tatian to John Paul II, through Jerome, Augustine, Bernard of Clairvaux, Thomas Aquinas, Luther, Calvin, and the great twentieth-century Reform theologian Karl Barth. Judgments on the theological meaning of sexual difference are tentative and unsettled for the first few centuries. Tatian seems to want us to ignore it as much as possible. Augustine, however, insists on the goodness and theological importance of sexual distinction, and his arguments on the subject have had an important influence on the subsequent tradition.

As Roberts explains, Augustine’s understanding was based firmly on the account of creation in Genesis. For Augustine, sexual difference is willed and created by the good and wise God. Though marred by sin, it is not the result of sin, nor is it created in view of sin and mortality: it will endure in the world to come. In this view, sexual difference was, even before the Fall, for procreation. Since it endures in the world to come, however, its significance must go beyond the good of procreation. Augustine believed that we must respond to the gifts of sexual difference and marriage by disciplining the concupiscence that, because of the Fall, distort them—and by respecting ourselves as the males and females God created us to be. Augustine understands Christian marriage from the perspective of Ephesians 5, which compares it to the relation between Christ and the church; and this understanding, too, will become an important feature in the subsequent tradition.

Luther continues the Augustinian vein, although, unlike Augustine, he does not recognize celibacy as a proper way to respond to sexual difference. We must each be as God made us. It is not our prerogative to be alone: never a man without a woman, never a woman without a man, and for Luther this means everyone must marry. Marriage, then, is not simply about managing lust or procreation; it is the adequate response to the good creation of sexual difference by the good God.

It is Barth especially who shows that sexual difference is for more than procreation. For Barth, sexual difference allows us to become an image of the Trinity, understood as a relation between persons. Furthermore, in being for one’s spouse in marriage, one thanks, honors, and obeys God. In this way, one is for God, just as Christ was for his Father in being for us. Marriage enables both spouses to be individually Christ-like in a family that, as a whole, mirrors the Trinity. “By the divine likeness of man in Genesis 1:27–29,” Barth writes, “there is understood the fact that God created them male and female, corresponding to the fact that God himself exists in relationship and not in isolation.” This does not sexualize God according to Barth, but rather makes the noninterchangeability of the sexes a token of the noninterchangeability of the divine persons. For Barth, then, as for Augustine and Luther, sexual difference is for sexual procreation within marriage, but procreation does not exhaust its meaning. While in heaven “there is no giving and being given in marriage” for the sake of children, sexual difference remains a constitutive element in the way human beings realize themselves in becoming an image of the Trinity.

John Paul II continues in the path of Barth. The nuptial meaning of the body is an invitation—inscribed in sexual difference, not apart from it—to imitate Christ and with one’s spouse to mirror the Trinity. Roberts prefers Barth to John Paul II precisely because Barth does not specify the content of sexual difference. Roberts thinks John Paul’s specification of the “womanliness” of women is more a function of Marian piety than of Scripture, and he praises Barth for offering a strictly theological argument for the significance of sexual difference, dismissing appeals to experience as a theological extravagance. One can share Roberts’s admiration for Barth without agreeing with the author’s suggestion that the Catholic appreciation of Mary is beyond the warrant of Scripture. Then too, Roberts’s insistence on settling things strictly theologically will endear him neither to those who think experience can be a font of theology nor to those who see in natural law a basis on which to engage nonbelievers.

Roberts’s excavation of the tradition has contemporary relevance, as he shows by examining some arguments for the legitimacy of same-sex unions. The Protestant theologian Graham Ward thinks that sexual difference as spoken of in Scripture is valuable only as a trope—as part of a theological rhetoric that describes the relations between Israel and God, Christ and the church, but not as a real requirement for the actual bodies that enter into marriage. Ward thinks we must be able to speak of our beloved as sexually “other” in order to connect our sexual love to the love of Christ and the church, but one’s own spouse need not be sexually “other” in the more literal sense. Roberts raises some good questions about Ward’s dismissal of hierarchy, order, and the mutual dependence of man and woman, arguing that these things are the conditions for the mutual self-gift within marriage that Ward otherwise values.

As his enthusiasm for Barth might suggest, Roberts thinks that the theological significance of sexual difference extends beyond procreation, and this figures crucially in his response to Eugene Rogers. Rogers is influenced by Rowan Williams’s dictum that if procreation is optional within marriage, then sexual difference must be as well. On this view, a church that finds contraception morally legitimate simply has no good reason to forbid same-sex unions. By contrast, Roberts thinks that the complementarity of sexual difference, even without procreation, is reason enough to argue against same-sex unions. He rejects Williams’s dictum and sides with Barth, who both required sexual difference for marriage and, under the right circumstances, allowed for contraception. Roberts says that desire and choice should not trump the “givenness” of created sexual difference, and he is insistent that human desire itself does not always reflect divine wisdom. But if one allows desire and choice to limit the procreative dimension of sex by means of contraception, why should desire not also trump the requirement of sexual difference? Roberts does not address this question.

Some readers will find Roberts’s treatment of St. Thomas a little thin. Some will wish the author had at least mentioned The Expositions of the Psalms in his section on Augustine. But we should not be ungrateful for what he has given us. Roberts writes with a shrewd eye for our contemporary predicament. “We cannot imagine existing in our culture without the haven of an erotic partnership,” he writes, “because our capacity to belong together in more chaste ways is so limited.” Here, he faults our failure to make possible “a social life of lay celibacy.” He notes that it is not only advocates for same-sex unions who want to redefine marriage. “Reclaiming the theological tradition about sexual difference would entail not only a chastening word to the revisionist theologians but also a thoroughgoing revolution for almost all Christians.” Would we not, for instance, have to put some daylight between the public social life of Christians and contemporary youth culture as celebrated by the media? With this book, Roberts has tried to raise the standard of theological argument about same-sex unions, and in this he succeeds admirably.