If you look at even the Apostles, they didn’t understand the Gospel, at least the death, burial, and resurrection, part of the Gospel, UNTIL GOD opened their eyes.
Jesus told them flat out several times that He would suffer, die and raise again on the 3rd day. It wasn’t really that they didn’t believe, it was that they didn’t understand it.
You know who DID understand it?
The Scribes and Pharisees.
They didn’t believe, but they certainly understood, even though Jesus was much more cryptic with them than the Apostles, about Him claiming to raise again to life.
So much so that they petitioned and got a Roman gaurd to watch the tomb in case the disciples tried to make Jesus’ claims come true.
God had to open the Apostles eyes. They couldn’t by themselves.