When Jesus asked 'Believest thou this?', what exactly did Martha confess?
Martha confessed that Jesus is the Christ, the Son of the living God, who should come into the world. She did not confess that He would die for her and be raised from the dead; the text explicitly says it does not say that.
In John 11, after Jesus tells Martha, 'Thy brother shall rise again,' she replies that she knows he will rise again in the resurrection of the last day. Jesus then says, 'I am the resurrection and the life. He that believeth in Me, though he were dead, yet shall he live. Whosoever liveth and believeth in Me shall never die. Believest thou this?' Martha answers, 'Yea, Lord, I believe thou art the Christ, the Son of the living God, who should come into the world.' The teaching points out that she did not add, 'and die for me, and be raised from the dead'; it does not say that. Her confession is the same kind of confession the gospel of the kingdom precipitated: Thou art the Christ, the Son of the living God—the Messiah, the King Redeemer, the prophet and priest.
She saith unto him, Yea, Lord: I believe that thou art the Christ, the Son of God, which should come into the world.