Here are the primary mainline denominations, every one of which has seen long-term decline in membership:
Christian Church (Disciples of Christ)
In 1965, the CC(DoC) had 1,918,471 members. In 2012, the membership was 625,252, a decline of 67 percent.
Reformed Church in America
In 1967, the RCA had 384,751 members. In 2014, the membership was 145,466, a decline of 62 percent.
United Church of Christ (Congregationalist)
In 1965, the UCC had 2,070,413 members. In 2012, there were 998,906 members, a decline of 52 percent.
Episcopal Church
In 1966, the TEC had 3,647,297 members. By 2013, the membership was 1,866,758, a decline of 49 percent.
(Those numbers should be even lower, though, since those figures by the TEC include breakaway churches trying to leave the denomination.)
Presbyterian Church (U.S.A.) (PCUSA)
In 1967, the PC(USA) had 3,304,321 members. In 2013, the membership was 1,760,200, a decline of 47 percent.
United Methodist Church (UMC)
In 1967, the UMC had 11,026,976 members. In 2012, the membership was 7,391,911, a decline of 33 percent.
Evangelical Lutheran Church in America (ELCA)
In 1987, the ECLA had 5,288,230 members. In 2013, the membership was 3,863,133, a decline of 27 percent.
(Note: The ELCA was formally constituted in 1988 as a merger of the Lutheran Church in America, the Association of Evangelical Lutheran Churches and the American Lutheran Church.)
American Baptist Churches
In 1967, the ABC/USA had 1,335,342 members. In 2012, the membership was 1,308,054, a decline of 2 percent.
(Note: The ABC/USA has been able to stem its decline among white congregants by replacing them with African American and Hispanic members.)
Now let’s look at a few of the primary non-mainline denominations, almost every one of which has increased in membership since the mid-1960s.
Church of God in Christ
In 1965, the CoG had 425,000 members. In 2012, the membership was 5,499,875, an increase of 1,194 percent.
Presbyterian Church in America
In 1973, the PCA had 41,232 members. In 2013, the membership was 367,033, an increase of 790 percent.
(Note: The Presbyterian Church in America was founded in 1974 by conservative members of the Presbyterian Church in the United States who rejected that church's merger with the United Presbyterian Church in the U.S.A.)
Evangelical Free Church of America
In 1965, the EFCA had 43,851 members. In 2013, the membership was 372,321 , an increase of 749 percent.
Assemblies of God
In 1965, the AoG had 572,123 members. In 2013, the membership was 3,030,944, an increase of 430 percent.
African Methodist Episcopal Church
In 1951, the AME had 1,166,301 members. In 2012, the membership was 2,500,000, an increase of 114 percent.
Southern Baptist Convention
In 1965, the SBC had 10,770,573 members. In 2013, the membership was 15,735,640, an increase of 46 percent.
Lutheran Church-Missouri Synod
In 1965, the LCMS had 2,692,889 members. In 2012, the membership was 2,163,698, a decline of 20 percent.
The mainline churches are finding that as they move further away from Biblical Christianity, the closer they get to their inevitable demise.
Joe Carter is an editor for The Gospel Coalition
You are wrong about the LCMS. It is a very biblically sound denomination. It may be losing members, but departure from sound doctrine is not the reason.
I’d like to point out that between the dates mentioned, in the 70s, there was a massive schism in the LCMS seminaries and that did a real number on the number of congregations in the Missouri Synod.
Ultimately, the real Christians won out and the Synod is still faithful, but the numbers have never quite gotten as high as they were before that, if I remember correctly.