Assalaamu alaykum, Scholar. I used to teach my sister that magic does not exists. However, I think I was referring to so-called magic tricks, those where they make tricks with cards and split a human into two and so on. So when told my sister that magic does not exist, I think I was referring to that. Now, praise be to Allaah, I have a bit more knowledge, and I believe that magic does exist but with the permission of Allaah, and that it is a major sin. My question is: did I commit and act of disbelief for not believeing in magic when I had no knowledge of that? Do I have to repent, even though I now do believe that it exists (with the permission of Allaah)?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

There is no doubt about the fact that magic exists, and whoever denies its existence is denying a fact proven by the texts of the Quran and the Sunnah as well as reality. The Prophet, sallallahu ‘alayhi wa sallam, was affected by magic, as is authentically stated in Saheeh Al-Bukhaari and Saheeh Muslim. Al-Qaraafi wrote:

“Magic exists and it is real. It could cause the victim's death or change his character and behavior even if he does not touch it himself ... The Qadariyyah sect held that magic is not real ... However, the Quran, the Sunnah, and the scholarly consensus prove that it is real... As for the evidence from the Quran, Allah, The Exalted, says (what means): {... teaching people magic...} [Quran 2:102] What is not real cannot be taught ... It was stated in the authentic Sunnah that the Prophet, sallallahu ‘alayhi wa sallam, was once affected by magic, so he used to imagine that he had had sexual relations with his wives while he had not. [Al-Bukhaari and Muslim] It was also reported that a slave girl whom ‘Aa'ishah, may Allah be pleased with her, bought cast a magic spell on her. Magic was commonly known to the Companions, and they unanimously agreed that it existed long before the Qadariyyah sect emerged.” [Al-Furooq] 

For more benefit, please refer to fatwas 32619 and 18942.

As for your past denial of magic out of ignorance, this is not considered disbelief because the existence of magic is not among the facts of the religion that are necessarily known by all the Muslims. What is necessarily known is that whose knowledge is prevalent to the extent that all people know it regardless of whether they are people of knowledge or laymen. There is no doubt that the fact that something is stated in the Quran does not necessarily mean that all Muslims know it; many lay-Muslims do not read the Quran, and many of them do not understand its verses. Some of them may encounter misconceptions pertaining to understanding or interpreting the meanings of the Quranic verses, and the person who is ignorant of the real meaning of the verse and misunderstands it is excused for his mistaken interpretation.

Allah knows best.