Tafsir of Ya seen 36:64

"Enter it today": This is an imperative of degradation and humiliation, similar to His saying, the Almighty: "Taste! Indeed, you are the..." [44:49], etc.

That is, experience its heat on this day for which you did not prepare. Abu Muslim said: That is, become its sala (fuel/fire). And it is said: Sala means its fuel. Al-Tabarsi said: Be bound to the torment within it; the root of sala is luzum (adherence/being bound). From this comes the word musalla (the one who follows), used for the one who comes immediately behind the predecessor due to his adherence to his track.

"Because of what you used to disbelieve": Your continuous disbelief in the world. The ba (in bima) is for causality (sababiyyah), and ma is an infinitive particle (masdariyyah). The possibility of it being a relative pronoun (mausulah) is far-fetched.