The enormity of what he was doing was beyond their understanding. I think that this is similar to discussions about Mary. If she truly understood who he was and what he was going to do, why would she go with his brothers to bring him home.
The Apostles had recognized Jesus as the Son of the living God, but they did not understand he was going to sacrifice himself to save us. After the transfiguration the Apostles were still confused asking why hasn't Elijah come first. Jesus had told them what was going to happen, but it wasn't until it did happen that they understood.
Yet, all (except Judas)were saved men before Christ went to the Cross.
So, clearly their salvation was not based on what we have to believe to be saved, that Christ died for our sins and rose from the dead and is now at the right hand of the Father.
Their salvation was based on believing that Jesus was their Messiah and the Son of God (Mat.16:16)