I don't see how that explains why people are leaving religious institutions and citing non-belief as a reason. How does something written in the Bible explain what is occurring in the 21st Century?
It is prophesied in many places. In Daniel it is prophesied that people seek knowledge during the end time. God's truth against human fallacies is in the scroll given to Daniel.
2 Thessalonians 2:9-11 (NIV2011)
The coming of the lawless one will be in accordance with how Satan works. He will use all sorts of displays of power through signs and wonders that serve the lie,
and all the ways that wickedness deceives those who are perishing. They perish because they refused to love the truth and so be saved.
For this reason God sends them a
powerful delusion so that they will believe the lie,
Science (can be good can be evil) performs the miraculous signs. The seem-to-be scientific reasoning however turns out to be deceitfully fallacious. It thus forms a powerful delusion to keep people in captivity (thus responsible for the decline).
Sidenote: "God sends" means God allows in a sense of absolute sovereignty.
Jesus also warned that when the end time comes, Christians need to endure to the end to be saved.
Matthew 24:13 (KJV)
But he that shall endure unto the end, the same shall be saved.
Interestingly,
Daniel 12:4 (KJV)
But thou, O Daniel, shut up the words, and seal the book,
even to the time of the end: many shall run to and fro, and
knowledge shall be increased.
Daniel 12:4 (NRSV)
But you, Daniel, keep the words secret and the book sealed until the time of the end. Many shall be running back and forth, and
evil shall increase."
It is accurately put (prophesied) about the effect of the end time knowledge.
It is firstly summarized in Genesis,
Tree of Knowledge (can be good and can be evil)
Genesis 2:17 (NRSV)
but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall die.
Decline = they are doomed under the influence of Satan to depart from God as a result of gaining knowledge in a fallacious way (by eating the fruit on the Tree of Knowledge).