Importance of Amr bil Ma'roof and Nahi anil Munkar

عَنْ عَائِشَةَ رَضِيَ اللهُ عَنْهَا قَالَتْ: دَخَلَ عَلَىَّ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، فَعَرَفْتُ فِي وَجْهِهِ أَنْ قَدْ حَضَرَهُ شَيْءٌ، فَتَوَضَاً وَمَا كَلَّمَ أَحَدًا فَلَصِقْتُ بِالْحُجْرَةِ أَسْتَمِعُ مَا يَقُولُ فَقَعَدَ عَلَى الْمِنْبَرِ، فَحَمِدَ اللهَ وَاثْنَى عَلَيْهِ، وَقَالَ: يَا أَيُّهَا النَّاسُ إِنَّ اللهَ تَعَالَى يَقُولُ لَكُمْ مُرُوا بِالْمَعْرُوفِ وَانْهَوْا عَنِ الْمُنْكَرِ قَبْلَ أَن ْ تَدْعُوا فَلَا أَجِيبَ لَكُمْ، وَتَسْأَلُونِي فَلَا أَعْطِيَكُمْ، وَتَسْتَنْصِرُونِي فَلَا أَنْصُرَكُمْ فَمَا زَادَ عَلَيْهِنَّ حَتَّى نَزَلَ
(رواه ابن ماجة وابن حبان في صحيحه كذا في الترغيب)

Hadrat Ayesha (Radiyallahu anha) says, "Once, Rasulullaah (Sallallahu alayhi wasallam) had come to me. I recognized from his face that some important matter has arisen. Rasulullaah (Sallallahu alayhi wasallam) did not speak to anybody. He performed wudhu and went to the Masjid. I stood listening behind the wall of the room as to what he would say. Rasulullaah (Sallallahu alayhi wasallam) ascended the pulpit and after praising Allaah Ta'ala, he said, "People, Allaah Ta'ala tells you to continue calling towards goodness and to forbid others from evil before the time comes when you will call Me and I will not respond, you will call Me and I will not give you and you will seek My help and I will not help you." He did not say anything further and came down from the pulpit. 
Those who are lax in matters of Deen and want to oppose the enemy should pay special attention to this subject. The help for the Muslims and help of Deen is hidden in being firm on Deen. Hadrat Abu Dardaa (Radiyallahu anhu) - a great Sahabi - says that you should continue commanding the good and forbidding the evil otherwise Allaah Ta'ala will cause such an oppressor to rule over you that will not respect your elders, nor will he have mercy on the young. At that time, your pious persons will make dua' but it will not be accepted. You will desire help but there will be no one. You will seek forgiveness but you will not be forgiven.

(محمد: ٧)  يَا أَيُّهَا الَّذِينَ آمَنُوا إِنْ تَنْصُرُوا اللَّهَ يَنْصُرْكُمْ

O you who have Imaan! If you will assist (in the propagation of the Deen of) Allaah Ta'ala, Allaah Ta'ala shall assist you (in all your affairs) and keep your feet firm (against all opposition)
(محمد: ٧)

In Durr e Manthur there is a narration of Hadhrat Hudhayfah (Radiyallahu anhu), quoted in Tirmidhi. It states that Rasulullaah (Sallallahu alayhi wasallam) took an oath and said that you should carry on commanding the good and forbidding the evil, otherwise Allaah Ta'ala will inflict you with punishment such that you will make du'a to him and He will not accept.

Upon reaching this point, the reader should think that how much sins we commit. Then it will be known why our efforts are in vain and why our prayers are without effect. (We also learn) as to whether we are planting the seeds of our advancement or our destruction. Fazaaile A'maal