(And We shall set up the Balances of justice on the Day of Resurrection, then none will be dealt with unjustly in anything. And if there be the weight of a mustard seed, We will bring it. And Sufficient are We to take account.)21:47,
In the first verse, it was said: وَالْوَزْنُ يَوْمَئِذٍ الْحَقُّ ` (And the Weighing on that day is definite). It means that the weighing of good and bad deeds on the day of Judgment is due and true. There is no room for doubt in it. Here, the hint given is that let no one be deceived by the idea that things which usually get to be weighed or measured are things having some weight or heaviness. Human deeds, good or bad, have no body or mass which could be weighed. How, then, would deeds be weighed? First of all, the reason is that the authority of Allah is absolute. He is powerful over everything. Why then, would it be necessary that something we cannot weigh could not be weighed by Allah Almighty as well? Then, other than that, we have before us, in our time, countless new instruments which claim to weigh or measure anything in the world. They no more need the traditional balance or scales or rod or tip. Modern instruments can weigh what nobody ever thought could be weighed, air, electric current, heat, cold, and so many other things. A meter is all you need. Now, if Allah Almighty, the Maker of makers, in His perfect power, could weigh or measure human deeds - with or without a device - what is there in it which would make it to be something far out to believe? Leaving this aside for a moment, the Creator of the Universe does have the power to transform our deeds at some time into a material presence, even give it some shape or form (interesting that people would believe something like this happening in a Speilberg movie, yet squirm at the religious view of things as would come to pass). There are many Ahadith from the Holy Prophet ﷺ ` which confirm that human deeds will appear in particular shapes and forms during Barzakh (the post-death - pre-resurrection state) and Mahshar (Resurrection). Good deeds of a person will, in forms beautified, become the companions of the grave - and evil deeds would crawl all over in the form of snakes and scorpions. A Hadith says that a person who has not paid the Zakah due against his wealth, that wealth will reach his grave in the shape of a poisonous snake to bite him and say: I am your wealth, I am your treasure.
In this world the importance of an individual is gauged by the material splendour surrounding him, and his ability to influence people with his impressive words, and thus have a massive following. Since the preacher is not generally in such a happy position, his word is valueless in the eyes of worldly people, while the words of his opponents, who bask in material grandeur have a greater impact. But when Doomsday uncovers all the facts, the position will be just the opposite. Now, all the weight will be on the side of the Truth: Untruth will be divested of all its arguments and will be rendered valueless.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.