“But stand up and enter the city and you will be told what you must do.”
Obedience is part of the gospel. Through the gospel God establishes a covenant. A covenant details God’s part and man’s part. Saul would have understood this instinctively.
After the establishment of the church, God, Jesus, nor the Holy Spirit never told anyone what to do to be saved. It was always left to those to whom the task had been given: the church.