How did the Twelve know what Jesus wanted them to believe?
The Twelve knew what Jesus wanted them to believe only when He clearly spoke and revealed it to them. They could not assume or believe anything He had not yet said. Faith comes by hearing, and hearing by the Word of God, so they waited until He spoke before believing.
Les explains that people sometimes ask whether the Twelve knew Jesus was going to die and be raised, but you cannot assume what God is going to say. You have to wait until He has spoken it and then believe it, just as Deuteronomy 29:29 says the secret things belong to the Lord but the revealed things are for us. Romans 10:17 declares that faith comes by hearing, and hearing by the Word of God. During His earthly ministry, when Jesus spoke things the disciples or the Twelve were supposed to believe, He made it clear so they could believe it. He did not expect Peter and the Eleven to believe what He had not yet said.