Albert Barnes Commentary Matthew 10:27

"What I tell you in the darkness, speak ye in the light; and what ye hear in the ear, proclaim upon the house-tops." — Matthew 10:27 (ASV)

What I tell you in darkness, etc. That is, in secret, in private, in confidence. The private instructions which I give you while with me, proclaim publicly on the house top. The house top, the flat roof, was a public, conspicuous place. See 2 Samuel 16:22. See Barnes on Matthew 9:2.