Tawbah's practical mechanics were never seriously disputed within Islamic scholarship, but a related and much sharper controversy arose in the earliest centuries of Islam over what a grave sin, committed and not yet repented of, does to a person's standing as a believer, and the argument shaped the formation of Sunni theology itself.
The Khawarij, emerging from a faction that had broken with Ali after the arbitration at Siffin in 657, held that a Muslim who commits a grave sin (a kabira) thereby becomes an unbeliever outright. The Mu'tazila rejected full unbelief but also rejected the Khawarij's opposite, that the grave sinner remains simply a believer; they proposed an intermediate category, al-manzila bayna al-manzilatayn, "the position between the two positions," holding that an unrepentant grave sinner is neither a believer nor an unbeliever, a status this school made one of its five foundational principles.
The Murji'a, by contrast, held that judgment on such a person should be postponed (irja) to God alone, and that faith consists essentially in belief and confession rather than in works, so a grave sinner remains fully a believer whose sin does not touch the reality of his faith. This position fed into what became the mainstream Sunni Ash'ari and Maturidi settlement: that grave sin does not expel a person from Islam, that judgment belongs to God, and that sincere tawbah, when it occurs, is efficacious regardless of which of these positions on unrepented sin one otherwise holds.