'Secret' in the Bible

until they were filled (permeated, saturated) with every kind of unrighteousness, wickedness, greed, evil; full of envy, murder, strife, deceit, malice and mean-spiritedness. They are gossips [spreading rumors],

Now to Him who is able to establish and strengthen you [in the faith] according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery [of the plan of salvation] which has been kept secret for long ages past,