Baylor sociologist and professor of religion, Rodney Stark, was recently interviewed on why Evangelical Churches thrive and Liberal/Mainline churches suffer and ultimately close their doors.
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There are congregations within these [liberal/mainline] denominations that are growing very impressively, very quickly, very strongly. Guess what? They have evangelical clergy.
Ten years ago I did a little study that I'm repeating now. There are various evangelical associations, like the Willow Creek Association. People all over the country belong to these associations. They're non-denominational ministerial associations of evangelical orientation. If you look at their membership lists, you will find that there are Congregationalists and Episcopalians and Methodists and so on. So I took out the names of the so-called "mainline" clergy who belonged to these ministerial associations; these were ministers who belonged to "mainline" denominations but had studied at evangelical seminaries and were generally evangelical in their convictions. When I traced them back to their congregations, guess what? In these rapidly declining denominations, the rapidly growing congregations belong to these guys.
Thus, the answer is simple: Evangelical churches grow because they preach the Gospel of Jesus Christ (Acts 4:12); that's what being "evangelical" means, being about the Good News (euangelium) or Gospel of Jesus Christ.
What is the Gospel? People have fallen out of relationship with God, their Creator (Eph. 2:1-3). People were created to be in right relationship with their Creator, but because of our sin (immorality, pride, selfishness, etc.) we have offended God's holiness (even our "righteous" acts are filthy in God's eyes; Isa. 64:6). God cannot stand for His holiness to be offended and thus all sinners (you and me) stand condemned by God and worthy of punishment (Romans 3:23; Romans 6:23). Jesus came to save sinners by dying on the cross of Calvary (Romans 5:8), bearing the due penalty for the sinfulness of humanity (2 Corinthians 5:21), and rising 3 days as the conqueror over sin and death (1 Corinthians 15:56-57). Now every person who trusts Jesus for their salvation from their sin can be reconciled to God, their Creator (John 3:16). So too, entering in to a new life with Christ allows us to enter on a glorious journey with God (2 Cor. 5:17). It starts in this life and never ends for all eternity (Phil. 1:21). We enter into the kingdom of God (John 3:3). We serve in the kingdom and participate in the kingdom work now and forever!
What if you went to a baseball game, and nobody brought a ball? The players just stir around for two hours. I don't think you'd go back, would you? Likewise, when you go to church, but the minister doesn't bother to hold church because he wants to talk about Medicare or something, why go back? Well, people don't.
A liberal political platform is insufficient for the suffering of human souls. A church that takes God, sin, or Jesus Christ out of their sermons, mission, and purpose will come to nothing. We need Christ; we need the Gospel; we need to be reconciled to God; hence, we need Gospel-preaching, evangelical churches and pastors. If you don't come with the ball (the Gospel), you won't have a church in due time.
Here's a good reminder: