The teenager, who came to Austria with his mother and sister in 2013, had claimed he acted in self-defence but a jury unanimously rejected this argument in court in Vienna on Thursday.
The wife of the deceased – a father-of-two including a three-month-old baby - broke down in court after revealing that her husband had been singing a song to her on the phone when he was attacked.
The incident happened in March this year in the district of Liesing in Vienna after the teenager met the older man, also from Afghanistan, on an AMS language course.
A fight broke out between the two of them about music and Islam, with the teenager reportedly telling the 31-year-old musician that he did not listen to music that the Quran does not allow. The older man retorted by insulting the boy's parents and making disparaging remarks about the Quran.
The fight was broken up by the AMS teacher and other students but the teenager returned the next day with two knives, stabbing the victim seventeen times including fatal punctures to his lung and stomach.
In his statement to the police, the teenager said: “I can just deal with the insult to my parents, but religion is very important to me.”
The victim's wife told the court that the family had “fled here from these religious fanatics”, only for her husband to become a victim of a religiously motivated attack.
In his summary to the court, judge Norbert Gerstberger confirmed that the boy had ambushed the victim in a surprise attack, adding that Austria does not tolerate murders due to religious mania.
“This is unacceptable in our society,” he said, before sentencing the teenager to 12 years jail time.