How can a person be sure of the soundness of their faith when they are in great doubt?

Tribulations reveal the truthfulness of people's faith. Whoever's heart remains firm in the face of doubts and desires, their faith is firm. And whoever is affected by them, their faith is not truthful. The believer's faith increases through tribulations and acts of obedience, and tribulations do not harm them if their heart is pure. As for the heart darkened by sins, it is affected by tribulations. Doubt in faith negates faith. If the doubt is a whisper from Satan (waswasah), one should seek refuge in Allah and stop entertaining it. If it is a real doubt, one should review themselves and draw closer to Allah.