"It is those who do not believe in the life to come who are suffering torment as they have gone far in error."
The torment may refer to their punishment in the life to come. Since it will inevitably overtake them, then it is as if they are already in it, just like they have gone irremediably into error.
But the statement may be understood in a different way, which suggests that those who do not believe in the life to come live in torment just as they live in error. This is a profound statement. A person who spends his life without belief in a second life suffers mental torment, as he lives without hope of justice, fair reward or compensation for what happens in his life.
Indeed human life is full of situations and trials which man cannot face properly unless he looks up with hope for justice and reward for good action and punishment for those who do evil. There are things that one cannot do or bear without looking up to God, hoping to earn His pleasure in the life to come, when nothing large or small is overlooked. Whoever is deprived of this window of hope, which brings comfort and satisfaction, undoubtedly lives in torment as well as in error. Such a person suffers all this in the present life, before suffering punishment in the hereafter for his misdeeds which brought about his present life's suffering.
"In The Shade of The Quran" - Sayyid Qutb, Vol. 14, p. 139