In Ephesians 3, Paul speaks about the stewardship, the household, that has been given to him (:2). He says that the secret has been made known to him by revelation and that he had already written briefly about it earlier (:3). Next, he writes:
Ephesians 3
4 by which you, as you read, can understand my insight into the mystery of the Christ,
5 which in other generations was not made known to the sons of men, as it was now revealed to His holy apostles and prophets.
apokalupto
The word translated here as revealed is the Greek apo-kalupto (G601). This word indicates the becoming visible of what was first covered. It was already there, but hidden.
This can, for example, refer to what is in the heart of man:
Luke 12 NBG
2 There is nothing covered that will not be revealed, and nothing hidden that will not be made known. Or concerning the day on which the Son of Man is revealed, when He appears (cf. Rev. 1:1):
Luke 17 NBG
30 In the same way it will be on the day when the Son of Man is revealed.
These examples show that revealing is making visible what was hidden.
The word apo-kalupto is composed of:
• apo (G575) = from
• kalupto (G2572) = to cover, to conceal
Literally: to remove the covering.
We find a clear example of this in 2 Corinthians 3, where kalupto occurs several times:
2 Corinthians 3 NBG
13 entirely different from Moses, who put a covering over his face, so that the children of Israel would not cast a glance at the end of what was to disappear.
14 But their minds were hardened. For to this day the same veil remains over the reading of the old covenant without being removed, because it disappears only in Christ.
15 Yes, to this day, whenever Moses is read, a veil lies over their hearts,
16 but whenever someone turns to the Lord, the veil is removed.
uncovered
This is not about an explanation of 2 Corinthians 3, but about what this illustrates. Israel has a veil when reading the old covenant. In Christ, that is removed.
Thus Christ Jesus made known to Paul by revelation the secret (Eph. 3:3-4). What was hidden was uncovered: the veil was removed.
as revealed now
When Paul speaks of the secret of Christ in Ephesians 3:4-5, he does not say that it was not there before, but that it has not been revealed in this way before. It was there, but hidden, for example in types and shadows of the Tanakh.
Ephesians 3
5 that in other generations was not made known to the sons of men, as it was now revealed to His holy apostles and prophets.
enlightened eyes
The covering has been removed. Now we see the old covenant without a veil. In it we recognize, in images and types, what Paul has revealed: the secret of the Christ (Eph. 3:4). We see with enlightened eyes of the heart (Eph. 1:18).