By Sarah Mac Donald - 10 February, 2014
Another six people were injured in the attack on the island which lies off Russia’s eastern coast.
A 24-year-old man has been arrested and is currently being questioned by police.
No motive has been given for the deadly attack and the suspect is believed to work for a local private security firm.
According to Orthodox priest, Fr Vicktor Gorbach, the gunman entered the cathedral shortly after a service had ended and began shooting at parishioners and religious icons on the wall of the cathedral.
Speaking to LifeNews, Fr Gorbach said there were not too many people left in the cathedral at the time and some managed to flee.
However, the nun and the male parishioner tried to stop the attacker and were killed. The priest said the man, who also destroyed a cross, expressed his hatred of the church.
In Moscow, the head of the Russian Orthodox Church described the nun and the man as heroes.
According to the Interfax news agency, Patriarch Kirill, who is based in the Kremlin cathedral, said “Those who died, died in the temple of God.”
“They tried to prevent that person from defiling our sacred place. They died as heroes, as soldiers on the front line.”
Patriarch Kirill said the attacker may be mentally ill or may have been influenced by those who speak ill of the church.
The Russian Orthodox Church has been criticised for its closeness to President Vladimir Putin.
The shootings occurred as Russia is on high alert because of the Winter Olympics in Sochi, but there was no apparent connection to the games. Sakhalin Island is about 7,500kms from Sochi.
Separately, on Sunday, Pope Francis greeted the organisers and athletes taking part in the Games in Sochi.
Addressing pilgrims gathered in St Peter’s Square in Rome, the Pope offered his best wishes that the Games might be “a true festival of sport and of friendship.”