I know a non-Catholic who through study became interested in Catholicism, but the Church he found nowadays barely resembled the Church in past times he had come to know–he said it seemed today like the ruins of a once great city. I immediately thought of the Third Secret of Fatima. I see the secret as more symbolic and as more related to a loss of faith, the death of so many vocations, the degradation of the liturgy in so many places, etc. (especially in light of other approved revelations, like Akita, about that same thing). This is why in 1960 it seemed so out of place–the Church was flourishing in these ways in an almost unprecedented way then.
It could also refer to Communist persecution. The ultimate error of Communism is denying man’s supernatural vocation, end and destiny and attempting to replace it with seeking an earthly utopia. Communism led to the physical persecution and death of many, but this error has also destroyed the faith and supernatural life of many.
In any event, the ultimate core of the secret is “penance, penance, penance” (especially since, despite this, the Church after 1960 has done away with much penitential disciplines and the sacrament of penance has been neglected everywhere) and to persevere up the mountain to God, praying for those along the way, despite the devastation and attacks (both physical and spiritual).