[18].Surah Al-Kahf [The Cave]: Ayat 98-99

Ayat 98. Dhul-Qarnain) said: "This is a mercy from my Lord, but when the Promise of my Lord comes, He shall level it (this wall) down to the ground. And the Promise of my Lord is ever true." 
Ayat 99. And on that Day [i.e. the Day Ya'juj and Ma'juj (Gog and Magog) will come out], We shall leave them to surge like waves on one another, and the Trumpet will be blown, and We shall collect them all together.

[Tafseer] After all the effort which Dhu al Qarnain has made for their protection, he claims no credit for himself beyond that of discharging his duty as a ruler. He turns their attention to Allah, Who has provided the ways and means by which they can be helped and protected. But all such human precautions are apt to become futile. The time must come when they will crumble into dust. Allah has said so in His Revelation; and His word is true. And so the spiritual lesson from the third episode is: Take human precautions and do all in your power to protect yourselves from evil. But no protection is complete unless you seek the help and grace of Allah. The best of our precautions must crumble to dust when the appointed Day arrives. And so we pass on to the Last Days before the Great Summons comes from Allah. All human barriers will be swept away. There will be tumultuous rushes. The Trumpet will be blown, and the Judgment will be set on foot.

Hazrat Zainab (May Allah be pleased with her) reported: The Messenger of Allah [SAWW](PBUH) came to visit me one day frightened and he [SAWW](PBUH) said, "La ilaha illallah (There is no true God but Allah). Woe to the Arabs because of an evil which has drawn near! Today an opening of this size has been made in the barrier restraining Ya'juj and Ma'juj (Gog and Magog people).'' And he [SAWW](PBUH) made a circle with his thumb and index finger. I said, ``O Messenger of Allah! Shall we perish while still there will be righteous people among us?'' He [SAWW](PBUH) replied, "Yes, when wickedness prevails".

[Al-Bukhari Book 04, Chapter 55, Hadith # 565].

Lesson : as mentioned above in Surah Al-Kahf Ayat 98. "the Promise of my Lord comes, He shall level it (this wall) down to the ground". This Hadith also tells us about the ominous of sin. It says that when vice becomes common, then in spite of the presence of some virtuous persons, that community will be destroyed. But on the Day of Resurrection, those noble persons who had done their duty of preaching virtue and checking vice will stand apart from the sinners. Thus this Hadith induces one to keep away from sins.