Ed Stetzer is executive director of LifeWay Research, an evangelical research organization. The views expressed in this column belong to Stetzer.

(CNN) The headlines were deafening this week -- if current trends continue, the last Christian at Boston's historic Park Street Church will leave the faith in a few decades, join a Wiccan coven on Harvard Square, tell her live-in atheist boyfriend that Christianity is dead, and we'll all just move on from this failed Christian experiment.

The headlines were reporting on a study from the Pew Research Center. This trove of religion data has some Americans acting as if the sky is falling on Christianity. But are the headlines accurate? Or did the numbers say something else?

A better reading of the stats is found when you move beyond the headlines and see a long, slow (but accelerating) decline of (mostly) nominal Christianity. However, the percentage of convictional Christians has remained relatively steady, with some decline. The decline of nominal Christianity is not the only thing happening, but it's a big part of the real story.

'Christian' means different things to different people

It's helpful to statistically clarify Christianity in the United States into three categories—cultural, congregational, and convictional. The first two categories are nominal Christians—they identify, but do not shape their lives around the Christian faith.

Cultural Christians are the least connected -- they call themselves Christian because of heritage or culture. Congregational Christians have some connection to a local church, but rarely attend. On the other hand, convictional Christians call themselves Christians like the other two categories, but they attend church services regularly and order their lives around their faith convictions.

If you read the headlines this week, you'd think the latter category is collapsing. But, that would be a sign of bad math, not an accurate reading of the situation.

About 70-75% of the U.S. population calls itself Christian, but about 25% of the U.S. population practices that faith in a robust manner. This includes, in order of size, evangelicals, Catholics, mainline Protestants, and others.

Yet, the majority of people who use the term Christian, do so in a way that is different from their church-going and faith-driven friends. Think about it—your religious uncle and irreligious neighbor could both call themselves Christians, but mean different things. I am not deciding here who is and who is not a Christian, but I am saying that it means different things to different people.

And, what they mean is changing.

What do the numbers say?

Today, we are seeing cultural Christians, and even some congregational Christians, now self-identify as religiously "unaffiliated." Folks who previously marked "Christian" on a religious survey because they weren't Hindu or Jewish are now choosing "none of the above."

In other words, nominal Christians (cultural and congregational) are becoming the "nones." That's not all that is going on, but the nominals becoming the nones are a big part of it. How do we know? Well, math.

If, in a given time frame, people calling themselves Christian declined by a large percent (as in the Pew data) and those who are committed to their faith and church declined at a much slower rate, the numerical difference between all Americans who call themselves Christians, and those who are active and committed, is important to note.

As Pew's Conrad Hackett explained, "people with low levels of religious commitment are now more likely to identify as religiously unaffiliated, whereas in earlier decades such people would have identified as Christian, Jewish or as part of some other religious group."

Yet, while the nominals becoming the nones is a key to understanding the situation, it's also important to know that the many of the convictional are remaining committed, particularly among the evangelicals.

Church attendance data and the level of religiosity are important here. Using Gallup's numbers, church attendance is about where it was in the 1940s. What's more, when we look at the General Social Survey, the best-known ongoing source of such data, we also find a rather stable, but slightly declining, percentage of the Protestant population attending church regularly.

So, there has not been a huge drop in Protestant church attendance. Over the past 40 years, according to the GSS, the share of Americans who regularly attends a Protestant church has only declined from 23% to 20%.

Catholic and historic African-American churches should be included in any comprehensive big-picture analysis, but due to space limitations, I'll focus on mainline and evangelicals, and Protestants as a whole. This chart tells the story of Protestants, using the GSS:

Yet, the kind of Christians going to church has changed, particularly among Protestants. It's moved from mainline, to evangelical. In 1972, 9% of the American population was regular church-attending mainline Protestant and 8% was evangelical, according to GSS. By 2014, the roles had reversed: church-attending mainline Protestants made up 4% of the population, while evangelicals rose to 13%.

The future

So, what does the future hold? Simply look at the polls of the next generation. It's an imperfect view because religiosity and age tend to impact one another, but, according to a recent ARIS study, the number of convictional Christians looks pretty similar among college students. What has changed -- the result of that slow (but accelerating) death of nominalism -- is cultural and congregational Christians.

We can see that trend by looking at two kinds of Protestants, evangelicals and mainline, which tell a remarkably different story. The RELTRAD system for the GSS includes non-denominational Protestants -- who attend once a month or more -- with evangelicals, so we've done that here:

If convictional faith, reflected (imperfectly) here by robust church attendance -- going to church "nearly weekly," "weekly," or "more than once a week" -- is a measure, we have a rather obvious decline (see the Pew numbers) of people who call themselves Christians. But we also see relatively steady numbers among those who are most committed, and an increase among evangelicals over the last decade.

The newsworthy decline has to come from somewhere—and it comes (mostly) from nominals becoming nones.

So, the big story is this: convictional Christians are remaining relatively steady with a slight decline. The nominals (cultural and congregational Christians) are often becoming the nones; and the sky is just not falling (unless you are a mainline Protestant).

There are challenges that face Christians, and we should not think all is just fine. These are sobering numbers. Thus, my point is not to say things are going well, but to point out what the data actually demonstrates.

If we measure Christian faith with a connection to Christian church, this is not a collapse of Christianity, but perhaps we are seeing an outbreak of honesty in how nominal believers respond to survey questions.