Tafsir of Al-Hujurat 49:15

"Artabu" (doubted): The intransitive form of rābahu, used when something casts one into doubt accompanied by suspicion. The meaning is: they believed, then no doubt occurred in their souls regarding what they believed in, nor any suspicion toward the One they affirmed, acknowledging that the truth is from Him.

If you ask: What is the meaning of "then" (thumma) here, given that it denotes delay, whereas the absence of doubt must be concurrent with faith—as I have explained that faith conveys the meaning of trust and tranquility, the reality of which is certainty and the negation of doubt?

I say: The answer follows two paths:

1. The first: A person who has attained faith may be accosted by Satan or some misguider after their heart had been at peace, causing them to doubt and casting into their heart that which damages their certainty. Or, they may look upon a matter with an unsound perspective, fall into doubt, and persist in it, stubborn and seeking no way out. Thus, the true believers are described as being far removed from these destructive states. Its parallel is His saying: "Then they remained steadfast" (Fussilat: 30).
2. The second: Since certainty and the removal of doubt are the essence of faith, it is mentioned separately after the mention of faith to alert one to its status. It is conjoined to faith with the particle of delay (thumma) to signify its stability over long, extended periods, remaining fresh and new.

"And they strove" (wa-jāhadū): It is permissible that the object of the striving is implied, whether it be the warring enemy, Satan, or one's own desires (hawā). It is also possible that jāhadū is an intensive form of jahada (to exert effort).

Striving with the self may mean military expedition, or it may encompass all acts of worship. Striving with wealth may refer to acts like what Uthman (may Allah be pleased with him) did for the Army of Hardship, or it may encompass Zakat and all financial acts of righteousness in which a man burdens his wealth for the sake of Allah Almighty.

"Those are the truthful ones": Those who were truthful in their saying, "We have believed," and did not lie as the Bedouins of Banu Asad lied. Or, they are those whose faith is a faith of truth, a faith of reality, diligence, and steadfastness.

"Say: Would you inform Allah of your religion, while Allah knows what is in the heavens and what is on the earth? And Allah is Knowing of all things."