There seems to be a difference between a secret and a mystery. At the very least, the Bible seems to use the word
mystery in more than once sense. For example, there might be something that can be known but has been deliberately kept secret. Then there can be something that, even though stated, is not understood.
This is the only way I can understand what Paul is saying when he declares that the conversion of the Gentiles was a mystery—that it was not made known to the sons of men. We find this discussion in Ephesians, chapter 3.