After talking about the salvation we are receiving, and looking forward to, Peter takes a look back at those who initially wrote about it. The Old Testament prophets wrote under the inspiration of God. But that does not mean they always fully understood the message God gave to them. Peter says that they searched intently to understand the mystery of salvation that they wrote about. But it was kept hidden until it was fulfilled in Christ.
The joy these believers was experiencing was not just because of the presence of Christ. And it was more that the expectation of their final salvation. Their joy was enhanced because they could see the work the Holy Spirit was doing in them. They were not the same as they had been prior to coming to Christ. They were being transformed, and took delight in that.