Can the Almighty's saying, "And they think that they are guided," apply to a person who strives to abandon sins and perform acts of obedience?

A person who is sincere to Allah cannot be misguided. However, one might think himself guided while he is not, if he compromises sincerity or adherence [to the Sunnah]. The misguided person does not know the truth, like the ignorant, while the deviant (ghawi) follows his whims despite knowing the truth, like the oppressor. Among the causes of misguidance are misleading tribulations, where the misguided person believes he is guided due to his turning away from revelation, similar to the state of monks and Kharijites for whom their evil deeds were made appealing. As for those whom the message has not reached or who were unable to access it, they have a different ruling, for Allah does not punish anyone until proof has been established against them.