Summary:
وَ لا تَقُولُوا لِمَنْ يُقْتَلُ في سَبيلِ اللَّهِ أَمْواتٌ بَلْ أَحْياءٌ وَ لکِنْ لا تَشْعُرُونَ
And do not say about those who are killed in the way of Allah , "They are dead." Rather, they are alive, but you perceive [it] not. {1}
وَ لا تَقُولُوا لِمَنْ يُقْتَلُ في سَبيلِ اللَّهِ أَمْواتٌ بَلْ أَحْياءٌ وَ لکِنْ لا تَشْعُرُونَ
And do not say about those who are killed in the way of Allah , "They are dead." Rather, they are alive, but you perceive [it] not. {1}
This verse encourages martyrdom in the way of Allah and praises it laudably, saying that it is not death, rather it is life - and what a life indeed! Death in the way of Allah is a distinction which is desirable - not a thing to dislike or fear. By saying that the martyrs are alive, actually means that they have not become extinct or have perished. In commonly language death is the opposite of life and with death one becomes extinct. In this context, the martyrs are alive.
The aim is to cheer the believers by reminding them of this reality, in order that they should not grieve, should not be perturbed, and should not loose their hearts when death faces them or their dear ones in the way of Allah. The only thing that the relatives would be afflicted with, in such cases, is separation from their martyr for a few days, as long as they themselves are alive in this world. And this temporary separation is not a big problem - especially if compared to the pleasure of Allah, and to the bounties bestowed on the martyr, such as the pleasant life of the hereafter and the everlasting grace.
Footnote:
1. 2:154
Source:http://www.al-islam.org/quran-and-mourning-hussain-jafar-ali-asil/status-martyrs-quran