Paul warns the Thessalonians and everyone reading his letter then and today about Apostasy. Paul was concerned that many people were being taught falsehoods, by false teachers.
“Let no man deceive you by any means: for that day shall not come, except there come a falling away first…” (2 Thessalonians 2:3)
Essentially before Jesus Christ can come for a second time, there WILL be a falling away from His Gospel. The time when there exists no proper Authority on the earth is known as an Apostasy. After the death of Jesus Christ, His Apostles ran the Church of Jesus Christ until all of them were persecuted and finally killed. The Power and Authority that had been given to them died with them.