A 2014 Pew Study says that 23% of the U.S. adults have no religious affiliation. But of these, 61% confess a belief in a higher power or God. These people are called "nones." It shows that "nones" are becoming more secular. Atheists and agnostics are in this group too. The designation comes from the none of the above category in the polls.
John T. Elson wrote the Time article "Is God Dead?' 50 years ago. He said that some seminaries and churches were dispensing with the concept of God as divine. But when the mystical is left out of the God equation, what is really left of God?
That's not to say that he believed God was dead. He coordinated a huge amount of research before he write the article. The 6-page article covered theological disputations and how some people's faith had changed.
Reasons People Leave Christianity
Opulence erodes the feeling and thought of needing God and church. Riches, being comfortable, these things can be a distraction for people. This is why Jesus told the young man that told him he had obeyed the commandments from his youth to sell his stuff and give the money to the poor and follow him. The young man walked away sad. His heart was attached to his money.
Some people can handle money and put it in perspective better than others. Jesus knew that young man couldn't do it.
Some people have left because because they find it boring. Others because they say a church has become too flashy. Some can't put science and faith together and see they don't contradict each other.
I think we have to look into and beyond these reasons to conclude that we want to know God and that faith is important in our lives.
Dwight Longenecker posits in his article "American Atheistic Materialism: A Creed of Despair" that materialists are left with just material reality, whereas, Christians have the material and the spiritual because the beauty of nature, all of God's creation and the magnificence of God are theirs.
He goes on to say that what people need isn't the drive to make church more socially relevant, but returning to the deeper spiritual meaning. This is what people are hungry for.
Dwight Longenecker is quoted below from his article "American Atheistic Materialism: A Creed of Despair."
"return to mysticism and monasticism, contemplation, prophecy, poetry
and prayer. Only then will the metaphysical well up within the physical,
and only then will we turn from materialistic atheism—not by force of
argument, but by the irresistible experience of beauty."
There are still more reasons why people leave church, but the dominant materialism of the U.S.A. is certainly a prominent one. Longenecker is correct that it is a philosophy of despair. As he said everything alive in the material world will die and decay is its destiny, with only that to look forward to despair is inevitable, even if people try to hold back despair by more gain and fun.