And Allah Most High says:

“This is because those who disbelieve follow falsehood, while those who believe follow the truth from their Lord.”

[Muhammad: 3]

He thus describes the believers as following the truth from their Lord, and whoever follows the truth is in the right. The believers follow the truth from their Lord, so they are most deserving of being called rigorous investigators. If the believers are the rigorous investigators, and one of their characteristics is that their faith increases when His verses are recited to them, then those characterized by the opposite are not rigorous investigators in the sight of Allah and His Messenger. Rather, they are rigorous investigators in the estimation of their own fellows—just as the Jews, Christians, polytheists, and every other group are rigorous investigators in the estimation of those who agree that what they say is true.

He has also described the believers in His words, Most High:

“Whenever a surah is revealed, some of them say, ‘Which of you has this increased in faith?’ As for those who believe, it increases them in faith, and they rejoice. * But as for those in whose hearts is disease, it adds impurity to their impurity, and they die as disbelievers.”

[al-Tawbah: 124–125]

It is well known that someone whose beliefs and desires contradict the revealed verses is not increased in faith by them, nor does he rejoice at their revelation. Rather, they add impurity to his impurity.

And Allah Most High says:

“The believers are only those who have believed in Allah and His Messenger, then have not doubted, and have striven with their wealth and their lives in the cause of Allah. It is they who are the truthful.”

[al-Hujurāt: 15]

Thus, the one who is truthful in the belief expressed by “have believed” is one who does not