What did the Third Secret of Fatima contain?

Thank you for your question. While we don’t know infallibly, we can say almost with certainty (based on a number of things) that the Third Secret concerns apostasy from the Catholic Faith by people who purport to hold positions of authority in Rome.

First, the very words which come just before the undisclosed Third Secret are: “In Portugal the dogma of Faith will always be preserved… etc. [Third Secret].” Since these are the last words before the Third Secret, they imply that the Third Secret deals with the dogma of Faith not being preserved. 

Second, people who have commented on the Third Secret, including high-ranking members of the Vatican II sect – who purport to have read the Third Secret or to have acquired intimate knowledge of its contents (including Joseph Ratzinger, “Cardinal” Ciappi) – reveal that it has to do with the loss of the Faith.

"In the Third Secret [of Fatima] it is foretold, among other things, that the great apostasy in the Church will begin at the top." (Mario L. Ciappi, “cardinal” and household theologian to John Paul II, quoted in The Devil's Final Battle, p. 33)

Third, Our Lady said it would be clearer in 1960, and this was just after Vatican II was called – the very council which put into motion the major apostasy from the Faith we are all now living through.