What does the Bible say about the mystery of iniquity?

In 2 Thessalonians 2:7, Paul speaks of 'the mystery of iniquity' which is actively at work, indicating a secret or hidden aspect of sin that operates within the world before the second coming of Christ. This mystery represents the presence of sin and false doctrine infiltrating the church, leading to a great apostasy amongst those who claim to be Christians. It reveals that there will be a great falling away from the truth of the gospel as professing believers turn away from sound doctrine.