The first answer is: They're not. Yes, some Lutherans in some parts of the world are witnessing significant numeric decline in their communities of faith.
The reasons for this decline are multi-faceted. Some people blame secularization. Others say cultural accommodation. Some say they've simply lost their missionary zeal. Others (and to my mind the most interesting) say that weakening Christianity in general is a natural outcome of "weak" Christianity that gives itself away for the world (see Gianni Vattimo on this, who argues that secularization is in fact the fulfillment of the central Christian message, and prepares us for a new mode of Christianity).
But there are Lutheran churches all over the world that are dynamic and missionary, experiencing Spirit-led growth and communities multiplying daily.
Some of the fastest growing Lutheran churches include: The Ethiopian Evangelical Church Mekane Yesus; The Evangelical Lutheran Church of Tanzania; Protestant Christian Batak Church (Indonesia); Malagasy Lutheran Church (Madagascar); Andhra Evangelical Lutheran Church (India). Most of these churches are now larger than the Lutheran churches in historically Lutheran countries like Norway or Denmark, and even the United States.
Over 50% of the population of Namibia is Lutheran. There are over one million Lutherans in Papua New Guinea.
In other words, when we talk about "Lutheranism," it is not the monolithic Eurocentric entity so many English-speaking Lutherans assume. Quite the opposite.
Nevertheless, many of us find ourselves in mission as Lutherans in European or North American contexts. Here, where we live, Lutheranism is not growing. Not any kind of Lutheran.
So why? Well, it's also true that in the United States, although some Christian denominations are holding steady rather than declining, there's no Christian denomination that is actually seeing growth. The only group seeing real growth in the United States are the "Nones," unfortunately labeled as such although they are in fact quite a bit more interesting than all that.
There's at least one simple explanation for all of this. The average members of these faith communities are not involved in evangelization. In every place in the world where Christianity is growing, it is growing because regular believers are active in evangelization. It's that simple, and that complicated. And part of the reason regular believers aren't evangelizing is also simple: those of us who are clergy aren't doing a great job of equipping believers for evangelization.
As a Christian, I often say I'm a missionary who currently serves as a pastor. My self-identity is that of a missionary. I may or may not be a particularly good missionary, but it's my center.
The relationship between my call to mission and my call to remaining Lutheran is, well, complicated. Let me give you an example. Just the other day, I was added to a confessional Lutheran Facebook group. I survived there about eight hours before being ejected. I was ejected for not being confessionally Lutheran enough.
To be fair, I shared a rather radical book with the group by a weird Lutheran, Johannes Bugenhagen, the pastor of Wittenberg during the time of Martin Luther. Quick plug, Augsburg Fortress is currently offering his selected writings for the Kindle at a deep, deep discount. It makes for fascinating reading. I recommend it.
Humor aside, when I was bounced from the group, I realized I had invested a bunch of energy trying to talk about the Christian faith with Lutherans of a very different stripe than mine. They're Lutheran, I don't understand them, but we're still both ostensibly Christian. I'm thankful to them for bouncing me. It taught me something.
Honestly, although it's mildly interesting to me to talk with "quia" confessional Lutherans, what I'd rather be doing with my time is talking with folks who aren't Christian, and representing the Christian faith in that context. I'd rather be a missionary than a polemicist.
There are other Lutherans who genuinely care about mission. One of my favorites is the Lutheran Society of Missiology. They strive to represent Lutheranism of all types and assist all of us in reflecting on God's mission and the missional mandate. That's not easy, when so many Lutherans are busy dividing rather than working together.
I like them because they believe that mission requires intellectual inquiry. Their journal, Missio Apostolica, attempts to do what is seldom done in Lutheran circles: articulate a Lutheran missiology.
It's surprising how little Lutheran missiology has been written. I really know of only one book written in the last decade, Mission Shaped by Promise: Lutheran Missiology Confronts the Challenge of Religious Pluralism (American Society of Missiology). And I can't say this one has been widely read or discussed, as promising as it is.
Similarly, the ELCA has a fantastic theological statement on mission, a missiological vision of accompaniment that, better than any other denominational statement I'm aware of, articulates the theological approach I myself am committed to as a missionary.
I tend to think that we are bad at evangelism in North America not because we have been culturally co-opted (quite the opposite, the ELCA as much as any denomination is ahead of the culture in gospel-focused ways), or because we have ineffective structures or something else. We are bad at evangelism because we have allowed simple barriers to evangelism to get in the way. We lack confidence. We're scared of evangelizing like other kinds of evangelism we resist.
In fact, it is not simply that we are shy. It is literally that we do not believe in evangelism. I remember sitting in on a class at a Lutheran college a couple of years ago where the whole theology class believed it was intrinsically colonialist to evangelize in any way at all. It's beyond that we are bad at it. We're actually actively against it.
We can do much better, both practically, and as I have indicated, theologically. For college students who are Lutheran to believe in evangelism requires an adequate theology of mission, one that inspires them.
Doing better would look like, at the very least, developing this more robust Lutheran missiology, spending more attention and energy on the missional mandate, learning from other communities who are doing evangelism well (especially global Lutheran communities), and attending to the new evangelization of charismatics and Roman Catholics and other Christian traditions that are growing globally.
It would consider Gianni Vattimo's insight that secularization is not a fail, but an opportunity to share together a new mode of Christianity. One of our great Lutheran theologians intimated something of this in his mention of religionless Christianity.
Above all, it would look like Lutherans giving up wholesale on insular infighting, and instead engaging the very wide and complex world that is now right at all of our doorsteps.