Kindness, gentleness and tolerance, not harshness, aggression and rebukes, are what open people's hearts to the message of truth. The Prophet (PBUH) used to advise the Muslims:

"Be cheerful, not threatening, and make things easy, not difficult."

Bukhari and Muslim. See Sharh al-Sunnah, 10/67, Kitab al-imarah wa'l-qada', bab ma 'ala al-walah min al-taysir.