The Miracle of Pentecost
Jesus' departing instructions for the disciples was to stay in Jerusalem until they received the power and indwelling of the Holy Spirit. They most likely didn't know the extent of the power that they would be given, and didn't know what it would be like when they had it. They knew that they would know. On the day of Pentecost, the Holy Spirit descended on the church and unleashed the power of heaven on earth.