Tafsir of Al-Munafiqun 63:3

(That) is a reference to what preceded of the statement denouncing them as being the worst of people in deeds, or to what was mentioned of their state in hypocrisy, lying, and shielding themselves with false oaths, or [their] superficial faith. There is in it a sense of distance, despite the proximity of the time to that which is referred to, due to what has passed repeatedly regarding the indication in such a context of the remoteness of their station in evil. Ibn Atiyyah permitted it to be a reference to the evil of what they did, so the meaning would be: their work became evil.

(Because they) meaning: due to the fact that they (believed), that is, they uttered the word of testimony like everyone else who enters into Islam, (then disbelieved), their disbelief became apparent and was clarified by what was revealed of their statements: "If what Muhammad says is true, then we are donkeys," and their statement during the Battle of Tabuk: "Does this man aspire for the palaces of Chosroes and Caesar to be opened for him? Far from it!" and other things. And ("then") is taken by its literal meaning, or to express the vast difference between the two states, or "then" they concealed disbelief, so "then" is for the sake of expressing distance and nothing else, or they uttered faith in the presence of the believers, then uttered disbelief in the presence of their devils in mockery of Islam. It has been said: The verse is concerning the apostates among them.

(So a seal was set upon their hearts) until they die in a state of disbelief, (so they do not understand) the reality of faith at all.

Zayd ibn Ali read ("Fa-taba'a" - so He set a seal) in the active voice, which is the pronoun of the Exalted One; and it is permitted that it be a pronoun referring to the verbal noun understood from what preceded, meaning: so it—their mockery of the religion—sealed it. In one narration, he read "Fa-taba'a Allahu" (So Allah set a seal), explicitly mentioning the Majestic Name; Al-A'mash read it in the same manner.