Ma'arif al-Qur'an
Mufti Muhammad Shafiفَلَيَعْلَمَنَّ اللَّـهُ الَّذِينَ صَدَقُوا (So Allah will surely know the ones who are truthful - 29:3). It means that through tests and hardships the good and the bad, and the sincere and non-sincere will be differentiated, because if the hypocrites are mixed up with the sincere believers and are not identified, it may create a number of problems. The object of this verse is to elucidate the difference between the good and the bad, and between the sincere and the non-sincere. The expression used for this purpose is that Allah Ta’ ala will find out who are the true ones and who are the liars. Since He knows about every one even before his birth, whether he is a true one or a liar, the sense carried by this expression is that tests and trials are conducted to bring the distinction of good and bad on surface, so that others should also know.
Hakim-ul-Ummah Thanawi (رح) has copied the argument of his Shaikh, Maulana Muhammad Ya` qub (رح) that sometimes people are addressed by descending down to their level of intellect. The common man makes distinction between the sincere and the hypocrites by testing him out. Therefore, according to their approach of understanding Allah Ta’ ala has said that, through various means, We would find out who is sincere and who is not, even though He knows everything from the very beginning.