I asked Abu Abdullah (asws) about the least with what the human (insaan) would happen to be a polytheist (mushrik) with. He (as) said: The one who begins to form an opinion (ibtidaa ra’ayan), so he loves upon it (ahib) or hates (abghaz) upon it.
[Tafseer Ayyashi, 1:150/246]
Regarding the verse from Holy Quran: “And who is more erring than he who follows his low desires without any guidance from Allah?” (Holy Quran 28:50), Abu Abdullah (as) said: It means the one who takes his religion by his own opinion without the guidance from the guiding Imams (as).
[Basair Ul Darajat Chapter 8]
Never form a personal opinion in matters of religion. You either know religious matters (through hadith) or you don’t. When there are no hadith on an issue we must refrain from forming any opinion on it. Unfortunately, the mujtahids violate this principle through ijtihad when they produce their risalah usually titled as “The fatwas according to the opinion of such and such”. People follow these mujtahids and their opinions and then they either love or hate other groups which agree or disagree with them.
Whenever a religious statement (fatwa) is issued or a religious practice is established that is not based on hadith it becomes a personal opinion in religion. This leads a person to become a polytheist (Mushrik).
Also read related article: Fatwa based on personal opinion