I didn't learn the term Mainline Protestant until my seminary days. During seminary, I increased applied dexterity of its usage, as I observed professors and students analyzing, discussing and lamenting both perceived and documented denominational relationships, strengths, weaknesses, cultural influence, history, theologies and mission partnerships.
Calling the denominations of Mainline Protestantism "Mainline," is similar to identifying myself an athlete, even though I haven't been a competitive athlete since the 1990's. My ability level has changed ever since I needed reconstructive knee surgery in 2004. I was an athlete at one time, but I would be delusional to call myself an athlete now, even though I still participate in fitness. I still put together enough training to race or prepare my body for a physical test of some sort, but I am no athlete. The fact that I am no longer an athlete does not deny my humanity. I still need to practice faithful stewardship of the body God gave me--I have other endeavors in which to serve God that require energy, thought, discernment, articulation, concentration, and occasionally a need to run. My body still needs to be at its best. But I am not an athlete.
The nomenclature of "Mainline" remains stuck in both academic and ecclesiological parlance. This usage is also delusional much like any self-identification as an athlete. Just because the denominations of the Mainline have been in decline for the better half of a century does not take away that these denominations are still part of the body of Christ. They still have an opportunity for faithful service in the name of Christ. Like a professional athlete, it's challenging to give up the broad social influence it once had. Professional athletes retire over and over again, hoping to regain a sliver of influence they once had, because it feels good (ask Brett Favre and Michael Jordan). Mainline Protestant denominations still approach their contexts recognizing their denominations have been in decline for decades, but energy is focused on who or what is to blame. A sense of entitlement of broad social influence rings in writing and speeches of Mainline Protestant denominational leaders--this entitlement showed in worship wars and the living on the back end of the church growth movement, as well as today's battles over human sexuality. Denominations of Mainline Protestant traditions continue trying to catch lightning in a bottle, hoping to grow the church. We love to stack our delusions on top of each other.
I do think my Mainline Protestant colleagues are turning a corner. Maybe we're progressing through the stages of grief and embracing a spirit of faithfulness. Growing up in the Pacific Northwest, I never understood the culture of broad social influence of Mainline Protestantism. I saw remnants of this during my years in the Midwest, but like any individual or group in recovery, there's a desire to name the hurts of the past. I still think "Mainline" is delusional. I've tried to create my own terminology to identify the past. I tried "20th Century Brand Protestantism," which receives the occasional courtesy laugh, but doesn't enter into conversations. Mainline Protestants are in recovery, and in the midst of discovering how God desires us to participate in the body of Christ and follow through in faith, we'll occasionally need to talk about what was. As long as we recovering Mainline Protestants aren't delusional about our applications and place in society, we can call our collective traditions what we want, and I'll stop pulling out my hair. I can't afford to do that, anyway.