Why preach to others even when there seems no hope.

Surah Al-Araf (7) , Verse 164:

وَإِذْ قَالَتْ أُمَّةٌ مِّنْهُمْ لِمَ تَعِظُونَ قَوْمًا اللَّهُ مُهْلِكُهُمْ أَوْ مُعَذِّبُهُمْ عَذَابًا شَدِيدًا قَالُوا مَعْذِرَةً إِلَىٰ رَبِّكُمْ وَلَعَلَّهُمْ يَتَّقُونَ

When some of them said: "Why do ye preach to a people whom Allah will destroy or visit with a terrible punishment?"- said the preachers:" To discharge our duty to your Lord, and perchance they may fear Him."

(English - Yusuf Ali)