The "Mystery Of Iniquity"?

In KJV 2 Thessalonians 1:7 the apostle Paul mentions the "mystery of iniquity".

KJV 2 Thessalonians 1:7, For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.

What is the "mystery of iniquity" in KJV 2 Thessalonians 1:7?

Could the "mystery of iniquity" be the ancient Luciferian organization called the "Illuminati"?