Christ taught some things openly to the crowds (Matthew 13:34-35) but other things were said privately to the disciples (Mark 4:34; Luke 24:27). Not that he had some secret message to deliver to his disciples which was different to what he said publicly (John 18:20).
In future, they are to preach upon the housetops – a figure for the most bold and public proclamation of the gospel message. There is one word that dominates evangelism – ‘proclamation ‘. The things he says must be made open and proclaimed. Therefore you cannot say I will work through drama and music, and adopt the style of the world, and then when they are softened up, I will bring in the gospel. No, says Christ, proclamation. It must always be direct.