The most consequential line in this creed is a refusal. Al-Tahawi states that no Muslim may be declared an unbeliever on account of a sin, however grave, unless that person holds the sin to be lawful. Behind that sentence is the first and bloodiest theological argument in Islamic history.
The Kharijites, who broke from Ali in the seventh century, held that grave sin put a person outside the community, with the consequences for life and property that followed. The Murjiites answered that judgement should be deferred to God and that faith remains faith whatever the sinner does. Between them lay the practical question of whether Muslims might lawfully kill Muslims over conduct, which was not an abstraction to anyone who lived through the early civil wars.
The Sunni settlement that al-Tahawi records takes the middle ground and gives it creedal form: sin is real, and it does not unmake a believer. The doctrine of takfir, declaring a Muslim an unbeliever, is hedged so tightly that in classical jurisprudence it is nearly unusable. That restraint is one of the things a creed can accomplish that a legal ruling cannot, because it puts the limit in the text that students memorise before they study law at all. The atlas records the classical jurisprudential position; how the term is deployed in modern polemic is a separate question and not the one this text answers.