In the simplest terms, whether in Greek or English, a mystery is a secret, something unknown. But wait. Much of the time, if not all the time, the mystery IS known to someone. A murder mystery is no mystery to the author (if fiction) or the perpetrator (if real life).
Twenty-seven times the New Testament speaks of “mystery” (or “mysteries”). Roughly half of these tell of the mystery being made known. A classic example is Col. 1:26: “the mystery which has been hid from ages and from generations, but now is made manifest to his saints.”
In Ephesians, Paul explains one of the specifics of the mystery: “By revelation he made known unto me the mystery… that the Gentiles should be fellow heirs… partakers of his promise in Christ” (Eph. 3:3-6). Salvation of the Gentiles was a mystery in the O.T. (Old Testament) because it was mentioned in such a way that even devout men like the apostles did not grasp it. Since the conversion of Cornelius (Acts 10), this truth is no longer a mystery. It has been clearly revealed.
Our Lord spoke of mysteries only once that is recorded; but this one time is very instructive. After hearing the parable of the sower, the disciples asked Jesus why He spoke in parables. Jesus replied, “Because it is given unto you to know the mysteries of the kingdom of heaven” (Matt. 13:11). The parallel text in Mark 4:11 reads “mystery” (singular).
“Know the mysteries of the kingdom of heaven.” Based on their perceptions of O. T. prophecies, the Jews expected a physical, national kingdom. Even Jesus’ 12 apostles, as devout as they were, did not see a spiritual kingdom predicted in the O. T. The full reality of these truths was hidden (a mystery) before Christ.
Today, because of the revelations of Jesus and the apostles, we look into the O. T. and see predictions of both the salvation of the Gentiles and a spiritual kingdom of God in general. But these were mysteries before Jesus began to unveil the true meanings. This is why the O. T. prophecies must be interpreted according to N. T. explanations. Many, though not all, of the O. T. predictions simply cannot be understood when read alone.
Herein, perhaps, lies the chief error of dispensationalism (futurism) today. Futurism believes that most O. T. prophecies, by themselves, are very plain and clear — to be understood literally. But how can futurists understand these prophecies by themselves if even the very prophets who gave the predictions did not understand them? “Of which salvation the prophets have enquired and searched diligently… unto whom it was revealed, that not unto themselves, but unto us they did minister the things” (1 Pet. 1:10-12). If the prophets did not understand, and the apostles (before Jesus’ death) did not understand, how can anyone today claim to understand all the O. T. prophecies by themselves. No, many of them cannot be understood apart from the explanations by Jesus and the apostles (following His death).
Mysteries remain mysteries until they are explained. Actually, they may continue to be called mysteries after being revealed, not because they continue to be mysteries, but because they once were. If these truths are not kept in mind — and applied — we will be as much in the dark on the meaning of many O. T. prophecies as the Jews in the first century were. When we examine any O. T. prophecy, one of our first questions ought to be: “Does the N. T. reveal important truths about this mystery?”