Should someone who boasts of his sins or does things that are seen by some scholars as Kufr (disbelief) doubt his faith? If the others are right, would he enter Hell forever? Please answer this question, because this would solve all my problems. May Allah, the One Who can punish sins or forgive them and Who is the Only One Who worthy of being worshipped let us live, die, and be resurrected as Muslims.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The person who boasts about his sins or commits sins, even if he does not boast about them, must repent to Allah, the Almighty, of his sins and of boasting about them, especially the sins that you referred to as seen by some scholars as Kufr. Repentance and giving up such sins are more emphasized given the danger of Kufr and what surrounds it, even if it does not take the doer out of the fold of Islam.

There is no doubt that the person's faith is decreased and weakened if he indulges in committing sins, whether major or minor; rather, some sins may amount to Kufr. The faith of such a person might be lost, may Allah safeguard us. If this is what you mean by your question about doubting one's faith, then its meaning is correct.

As to whether such a person would enter Hell forever, then the knowledge of that is with Allah alone. We cannot affirm that someone will certainly enter Paradise or Hell unless it is established in the Quran or authentic Sunnah. We are not required to do so; rather, we are required – as mentioned above – to repent from Kufr, Bid‘ah (religious innovation), and major and minor sins.

For more benefit, please refer to fatwas 276013 and 86527.

Allah knows best.