Jesus was meeting with the disciples in an upper room, behind doors which hid him from public scrutiny--at least for the moment.  His words flowed from His heart.  He said--probably quietly and with great confidence--"Stop letting your hearts be troubled!"  Verbs in Greek convey a definite kind of action that they do not convey in English, and the one that Jesus used indicated that worry and concern had begun a long while before and was continuing.  But He says, "Stop doing it."