The first depends partly upon the second, but we'll get to that. Now, the first answer to the question of how we can convert the heart of our culture, is that we can't. God can do it, and we can offer ourselves as His instruments; but if we approach it as a victory that we are aiming for, we will probably lose, and (more importantly) even if we win, the victory is likelier to corrupt us than bless us. For the more we think of it as a victory, and the more we think of ourselves as victors, the more our eyes turn inward, to look upon ourselves rather than our Savior. And the more we think of ourselves and of our victory, the more, too, we are disposed to trample and disgrace those against whom our victory was won. This is not just theoretical moralizing; its hideous truth is a fact of Christian history. "No, somehow we must be saved together."
Which means that, as far as our own duties are concerned, the first thing is -- what the first thing always is: trust in God, expressed and nourished through prayer and the life of the sacraments. These are the means whereby we lay ourselves open to the supernatural operation of the Holy Ghost, "who broods over this bent world / With warm breast, and with ah! bright wings." If we do not offer ourselves as victims of divine Love, then whatever else happens, it won't matter. And if we do, then whatever else happens, it will be to us a sacrament of that Love.
The second thing is ourselves to recover an authentic attitude toward marriage and toward the different kinds of love -- something which commanded our Pontiff Emeritus' attention in his first encyclical. This means taking marriage out of the center of our lives, hopes, and affections, and putting God there, as having probably the better claim. It means making a distinction (in thought, speech, and practice) between eros as a potent and beautiful experience, and the deliberate decision to build a life with someone, this latter being part of what marriage is with or without eros. And it means opening every decision we make, every institution we are a part of, and every relation we have to every person, to the movement of love-in-grace.
Other than that, I think the chief thing is for each of us to discern our own vocation (marriage, lay celibacy, priesthood, or consecration to a religious order) and live it to the hilt. Being ready to answer when we are asked what we believe is important; sometimes -- though less often than zealous and talkative Christians are apt to feel -- initiating a conversation on the subject is the thing to do; but every intellectual defense will ring hollow without a life lived as a sacrament of the grace of God. And by grace here I do not simply mean mercy: I give the word its full, Catholic force, as a participation in the supernatural life of the Trinity. If we wish to display that the natural and supernatural meanings of marriage are really true, then we need to show in our own persons, our own bodies, that they do what we say they do. And this is displayed not only in marriages lived in fidelity, fertility, and charity; it is also manifested in celibacy, not only priestly and monastic celibacy but that of laymen. A celibate life, lived in the grace of Christ and the communion of fellow believers, can be a vivid sign of the holiness and significance of the flesh, necessary prerequisites for any thoroughly Christian approach to marriage.
On this point in particular there is some room for improvement. When I was an evangelical, I found that, with a very few exceptions, I would be positively opposed and discouraged if I made bold to talk about being celibate. This trend is not universal, but it is common (especially in America, I gather); it is also radically unbiblical. Justin Lee pointed out in his excellent book,* Torn, that many churches have no tools at all for making celibate Christians either feel welcome or figure out how on earth they are supposed to conduct their lives: "American culture tends to obsess over romantic relationships -- just turn on the radio, watch TV, or go to the movies for proof -- and people who make it to midlife without having married must face the perception that something is 'wrong' with them. The church has the power to be a family to single people and to give them a place to feel fully welcomed and included. All too often, we fail to do that. Even when churches offer special classes or programs for single adults, many of them only consist of some combination of general Bible knowledge and teachings designed to prepare people for meeting their future spouse; few of them adequately address the unique needs of single people as single people." (Torn, p. 239)
In other words, don't be this guy:
The Catholic Church has a tradition of celibacy far stronger than that of most Protestant bodies; indeed, that is one of the things that drew me across the Tiber. But its devices are aimed primarily at priests and members of religious orders; celibates who do not find themselves called to these specialized vocations can find themselves feeling that they are neither beast nor bird, like the hapless bat in Aesop. Moreover, as a (heterosexual) female friend of mine said to me, the kulturkampf can make the Church seem almost like a commercial for marriage at times, exacerbating the difficulties that already attend a vocation to celibacy with a feeling of isolation and strangeness. I don't know that we need more in the way of structures to support single believers; what is really needed is clear teaching about the whys, hows, and whats of celibacy outside the priestly and religious paradigms, together with integration into the whole community of faith: married, priestly, religious, and lay celibate. Support for celibate Christians, both in pursuing their vocations, and in seeing that they are embraced and held up by the community around them, is vital. And the word community, remember, means that you and I should do something, not that other people should do something.
I had originally thought I'd be able to deal with the second practical consideration after a concise treatment of re-evangelizing our culture about marriage, but the giant wound of how Catholics interact with queer people is proving more difficult to dress than I'd given it credit. Once again, I defer it to my next.
*Some Christians may be puzzled or discomfited that I describe this book as excellent, when it defends and promotes a view of homosexuality that is categorically discountenanced by the Catholic Church. I see no more difficulty about this than about my deep love for C. S. Lewis and Charles Williams, who even at their most Anglo-Catholic dissented from the doctrine of the papacy, which I on the contrary treasure. Few things have so degraded the quality of intellectual discourse in our culture as the false idea that one must despise things merely because one disagrees with them -- a point, coincidentally, made in rather different language by Justin Lee in his book.