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]
We should not have personal opinion in matters of religion. We are either sure of a religious matter (through hadith) or we are not. When there are no hadith on an issue we must refrain from forming any opinion on it. 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).