Is there any perspective that, we can say/call Shia Muslim or Sunni Muslim? Is not it sin/bidah ?
Indeed, those who have divided their religion and become sects - you, [O Muhammad], are not [associated] with them in anything. Their affair is only [left] to Allah; then He will inform them about what they used to do. Surah Al-An'am Verse 159
And hold firmly to the rope of Allah all together and do not become divided. And remember the favor of Allah upon you - when you were enemies and He brought your hearts together and you became, by His favor, brothers. And you were on the edge of a pit of the Fire, and He saved you from it. Thus does Allah make clear to you His verses that you may be guided. [Ali Imran : 103]( https://qurano.com/en/3-ali-imran/verse-103/
Say, [O believers], "We have believed in Allah and what has been revealed to us and what has been revealed to Abraham and Ishmael and Isaac and Jacob and the Descendants and what was given to Moses and Jesus and what was given to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [in submission] to Him." Surah Al-Baqara Verse 136