If their defences are strong, it is because you have not used the weapon of mercy.
‘Were you rough and hard of heart they would have dispersed from round about you.’ (3:159) is the battlecry of Da’wa. ‘Allah gives through gentleness what He does not give through roughness’ (Hadith). The Holy Prophet ﷺ transformed his hard-hearted people in only twenty-three years. He took them from the many to the One, from vendetta to the Sacred Law, from despair over death to the certainty of eternal life. In this he was ‘sent only as a mercy to the worlds’ (21:107). But without his mercy this mercy would have been marked ‘Return to Sender’.