Is my mere assumption that the Law of Attraction exists considered a sin, even if I haven't read about it or applied any part of it, and while not believing in philosophies that deny divine decree (Qadar)?

It has been previously explained that the Law of Attraction contradicts Islamic creed because it has corrupt philosophical and dogmatic roots. There are studies about its relationship with magic and pagan beliefs. To understand its falsity, one can refer to Sheikh Abdullah bin Saleh Al-Ujairi's treatise, "The Hoax of The Secret: An Analytical Reading of the Book 'The Secret' and the Law of Attraction." If a believer finds the matter confusing, their duty is to refer to scholars and ask them, as Allah Almighty says: "So ask the people of the message if you do not know." (Quran 16:43). If they request legal evidence and rational proof, that is good. Once the truth becomes clear, they should believe in it and act upon it. It is understood from the questions that the questioner is afflicted with waswasa (whispers/obsessive thoughts), so we advise them to disregard it and not pay attention to it, and to repel anything that weakens faith. Indeed, Ibn Hajar al-Haytami mentioned that the beneficial cure for waswasa is to completely ignore it.