Is it true that declaring a person who has committed a major sin to be an unbeliever is considered an innovation in religion?

Excommunicating absolutely anyone who commits a major sin is the doctrine of the Khawarij. As for Ahl al-Sunnah wal-Jama'ah (the People of the Sunnah and the Community), they hold that one who commits a major sin is not an infidel and will not abide eternally in Hell, so long as they are a monotheist. They adduce as proof Allah's saying: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills," and the hadiths of intercession, such as: "My intercession is for those among my Ummah who commit major sins," and the hadith: "Jibreel came to me and gave me the good tidings that whoever dies not associating anything with Allah will enter Paradise. I said: 'Even if he steals and even if he commits adultery?' He said: 'Even if he steals and even if he commits adultery'," and the hadith of 'Ubadah: "And whoever commits any of these sins, and Allah conceals it, then his affair is with Allah; if He wills, He will punish him, and if He wills, He will forgive him." Accordingly, the assertion that the perpetrator of a major sin is an infidel is an innovation.