A friend of mine who is now Muslim, use to go to heavy metal concerts and put 666 on clothing. He now understands the evil of his past and is wondering if he could be forgiven from this ignorance of his past.

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We have many verses in the Holy Quran say that if a person has accepted the right path and not committed sin after becoming a believer his past sin will be forgiven. So about the forgiveness of Allah (swt) no one can give assurance but He himself has given and He is the Most Merciful and Most Forgiving.
May Allah (swt) keep us on the right path.