Olaf Wiig is probably >thisclose< to converting to Islam anyway. Without any any firm convictions of his own, having abandoned whatever religion he may have been introduced to early in life, he has been ripe for the picking by whatever cult got to him first.
It just happened to be some militant Islamic jihadists. If offered the opportunity, the conversion could just have well been to Druidism, or Wicca, or animism.
Sometimes secular humanism just is not enough to satisfy the spiritual hunger of the masses.