All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Dying as a disbeliever is the act that deprives man from ever entering paradise. Allaah the Almighty Says (what means):
• {Indeed, those who disbelieve and die while they are disbelievers - never would the [whole] capacity of the earth in gold be accepted from one of them if he would [seek to] ransom himself with it. For those there will be a painful punishment, and they will have no helpers.} [Quran 3:91].
• {Indeed, those who deny Our verses and are arrogant toward them - the gates of Heaven will not be opened for them, nor will they enter Paradise until a camel enters into the eye of a needle. And thus do We recompense the criminals.} [Quran 7:40].
As for a person committing any other sin, no matter how grave, he will eventually enter Paradise, though he may remain in Hell for some time, until he is punished for his sins. This happens if the person dies as a Muslim. In a Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, explained that when the people of Paradise enter Paradise and the people of Hell enter Hell, Allaah the Almighty, by His Mercy and Grace, will order that any person whose heart included the least amount of faith be taken out from Hell. [Al-Bukhari]
Allaah Knows best.