The one consistent feature among the so called "mainline" Protestant churches is the ever accelerating decline in their membership and influence.The statistics are clear.
The United Methodist 11 million 1968 - 8 million 2008
Evangelical Lutheran Church 1984 5.2 million - 3.9 million 2012
Presbyterian Church USA 2006 2.4 million - 1.8 million 2013
American Baptist Church, United Church of Christ, and Disciples of Christ all in decline.
What is the single characteristic All of the Churches have in common.They have all succumbed to liberal Theology and have progressively embraced it to a greater degree over the past eight decades.
Each of these Churches have accelerated in their decline as they have become progressively more heterodox. The Losses have either come from slow erosion as their young people walk or massive defections of members leaving to join more Orthodox denominations, or to start more Churches faithful to the historic Creeds and Confessions.
The interesting thing is that these Churches never seem to accept that it is their Theology,(or lack of Theology that is the cause of their continual decline. In the Presbyterian Church USA for example there has not been a year of growth in 30 years. In the United Methodist surveys have shown that the rank and file members of the denomination tend to be far more Conservative than the denomination. but the hierarchy simply ignores them, (Ironically the Denomination worldwide is growing because its third world have not embraced the Theological errors of the American Church. The Episcopal Church in spite of the defection of Thousands still continues to Ordain Homosexual and Women bishops.
Sadly, one even sees the trend in historically faithful Churches. The Christian Reformed Church has compromised on a number of issue such as Women's Ordination, leading to the separation of a number of it congregations to for the United Reformed Church's. Logic would argue that these Churches, if only to save themselves should return to Orthodoxy. But sadly their lack of Logic is not the problem. Theological Liberalism is an outgrowth of the fall, and sinful mans desire to throw of Gods law in the scriptures, To replace that standard with something more attuned to mans fallen nature. Therefore they cannot change. Much like the situation in the Scouting movement where the leftist agitators would rather destroy the organization if they can't corrupt it. As it says in the scriptures, "but he who fails to find me injures himself; all who hate me love death" Proverbs 8:36 ESV. and "There is a way that seemeth right to a man but the end thereof is death." Proverbs 14:12
We must grieve for the mainline Churches. In the case of my own Presbyterian Church they were once the spiritual homes of men like Dabney and Hodge, Warfield, Palmer, and Thornwell. We must grieve to for that small faithful and confused remnant still present there. We must well stand guard over own Churches to counter this same corruption.