السَّلَامُ عَلَيْكُمْ

Just like the opposite of tawheed is shirk, the opposite of ittibaa’ is bid’ah. Bid’ah is the worst of the major sins after shirk.
The root of the word bid’ah in Arabic means initiating something without any precedent.

In Islam, bid’ah means something that has been introduced into the religion of Allah that has no general or specific basis to support it.
There is a clear principle regarding bid’ah:
Everything in the religion is haraam, unless you have a proof to make it halaal, and everything outside of the religion(customary things) is halaal, unless you have a proof to make it haraam.

Bid’ah is so serious because:

1. A person will never repent from bid’ah, because they believe that they are doing good deeds

Say, [O Muhammad], “Shall we [believers] inform you of the greatest losers as to [their] deeds? (18:103)

1. It involves either:
   a. Implying that your method is better than that of the Messenger and his companions.
   b.Implying that the Messenger failed in his job to teach us everything in the religion,
   and betrayed the trust that Allah placed in him.
   c.Implying that the religion is not complete or suitable for all times.
   -It involves causing a sunnah to be lost.

Allah knows best.

Sources:

Lecture Notes:
https://www.digitalmimbar.com/AqeedahNotesTimHumble.pdf
-It leads to further misguidance and spreads misguidance amongst the people.