I make a distinction between the means to salvation, and the promise of salvation. The promise of eternal life was given all the way back to Abraham's day. What Abraham and the others didn't know is how God would accomplish this, i.e. death, burial, resurrection and ascension of Jesus Christ.
So I agree. The apostles didn't know that the messiah would need to die on a cross. But they did know that God promised them eternal life.
This tension can be seen in the passage concerning the death and resurrection of Lazarus. Notice that Mary and Martha both believe that Lazarus would be raised on the last day. What they didn't know was that Jesus would be instrumental in the process. John 11 "I am the way, the truth and the Life", he says. This would be new information for them.
Jesus proves his point by calling Lazarus out of the grave. In this way, Jesus will call all of his followers out of graves at the last trump.